What Is the Flu?

Influenza, a viral infection, causes an illness that can range from mild to life-threatening.

Influenza, more commonly known as “flu,” is a viral infection of the respiratory tract that affects the nose, throat, and sometimes lungs.

Outbreaks of flu tend to happen annually, at about the same time every year. However, each outbreak may be caused by a different subtype or strain of the virus, so a different vaccine is needed to prevent the flu each year.

For most people, a bout of flu is an unpleasant but short-lived illness. For others, however, flu can pose serious health risks, particularly if complications such as pneumonia develop.

Every year, thousands of Americans die from the flu. According to the Centers for Disease Control and Prevention (CDC), the number of deaths caused annually by flu in the United States ranged from 3,000 to 49,000 between 1976 and 2006, with an annual average of 23,607 flu-related deaths.

The best way to avoid getting the flu is to get an annual vaccination, encourage the people you live and work with to do likewise, stay away from people who are sick, and wash your hands frequently.

Types of Flu

There are three types of human influenza virus — type A, type B, and type C — and many different variants within those types.

Because several types or strains of flu can circulate simultaneously, each year’s flu vaccine protects against the three or four viruses predicted to be most common in the coming season.

Influenza A and B cause the seasonal epidemics of flu, with symptoms that can range from mild to severe. Two types of influenza A and one or two influenza B viruses are included in the seasonal vaccine.

Everyday Health Flu Map: Are You At Risk?

Influenza C causes a mild respiratory illness and is not thought to cause epidemics. It is not included in the flu vaccine.

H1N1, or swine flu, is a type of influenza A that infects pigs and can also infect humans. It is currently included in the flu vaccine.

Avian flu (H5N1) naturally occurs in wild aquatic birds and can infect domestic poultry. It rarely infects humans but can be severe when it does. Most cases in humans have occurred in Asian countries.

While some people refer to a brief episode of nausea, vomiting, and malaise as “24-hour flu” or “stomach flu,” these are unrelated to influenza and are usually due to infection with other viruses such as norovirus.

How the Flu Spreads

The flu spreads when an infected person coughs, sneezes, or talks, propelling droplets of liquid into the air that either land on another person’s mouth or nose or are inhaled into a person’s lungs.

The flu may also spread through direct contact with an infected person’s nasal secretions, or by touching an object with flu virus on it, then touching one’s mouth or nose.

Symptoms of flu typically start within one to four days of infection.

An infected adult may be contagious from one day before symptoms start to 5 to 7 days after becoming sick. Children may continue to be contagious for longer than 7 days.

Given how easily the flu is transmitted, staying home while sick and keeping sick children home are important ways to reduce the spread of the flu.

Flu Complications

Complications of the flu can include:

• Asthma flare-ups
• Bronchitis
• Ear infections
• Heart problems
• Pneumonia
• Sinus infections

Complications of influenza may be caused by the flu virus itself or by a bacterial infection, called a secondary bacterial infection.

When a patient is ill, it can often be difficult to determine if their illness is from the flu alone or a secondary bacterial infection.

If the infection is caused by bacteria, antibiotics may be used to treat it. If it’s caused by the flu virus, antibiotics will not be helpful in treating it, since antibiotics have no effect on viruses.

Sources:

• Seasonal Influenza (flu); CDC.
• What You Should Know for the 2014-2015 Influenza Season; CDC.
• Influenza (flu); Mayo Clinic.
• H5N1 Avian Flu (H5N1 Bird Flu); Flu.gov.

Symptoms of the Flu

Influenza symptoms typically include fever, chills, headache, muscle aches, sore throat, runny nose, and fatigue.

Flu symptoms often start with a fever that comes on quickly.

As the illness develops, fever may be accompanied by chills and sweats, muscle aches, headache, sore throat, cough, sneezing, runny nose, stuffy nose, and a general feeling of being unwell.

Occasionally the flu causes nausea, vomiting, and diarrhea. These symptoms are more common in children.

A less common symptom is losing one’s sense of smell for days, weeks, or (rarely) permanently.

The flu typically lasts for about two weeks, with the severest symptoms improving after two or three days. But fatigue caused by influenza can persist for weeks.

Cold versus Flu Symptoms

The common cold is caused by different viruses (e.g., corona virus or rhinoviruses) than the influenza virus, but the two types of infections have several symptoms in common, such as:

• Sore throat
• Fatigue
• Fever
• Sneezing
• Coughing
• Runny nose
• Nasal congestion

Symptoms that make a diagnosis of flu more likely than a cold include:

• More severe symptoms overall
• High fever (101.3 to 104 degrees F)
• Body aches
• Illness occurring during a flu outbreak

Flu Symptoms in Children

Flu symptoms in children are similar to symptoms in adults although children are more likely to experience nausea, abdominal pain, vomiting, and diarrhea. Young children may also be noticeably irritable.

In infants, symptoms may include lethargy and poor feeding.

Everyday Health Flu Map: Are You At Risk?

Children may be particularly bothered by nasal congestion resulting from flu because of their narrower nasal passages. In addition, infants and young children often find it difficult to breathe through their mouths.

In infants, nasal secretions should be gently removed with a rubber suction bulb before feeding times and before sleeping. Saline nose drops can be used before suctioning to loosen mucus in a stuffy nose.

Older children can be taught to blow a runny nose and can also use saline nose drops to loosen mucus in a stuffy nose.

Flu Without Fever

Occasionally people who are infected with the flu virus have no fever, at least not at the onset of other symptoms. Some people go on to develop a fever later, and some do not.

With or without a fever, a person who is infected with influenza is contagious and capable of infecting others.

Flu Test

The flu is usually diagnosed based on symptoms, especially if they occur during flu season. Occasionally, however, a doctor may take a swab of mucus from the nose or throat to test for influenza virus.

Some possible reasons for testing include:

• Ruling out the flu if a doctor suspects that something else is causing a person’s symptoms
• Confirming that a person has the flu so that antiviral therapy can be started quickly
• Tracking an influenza outbreak in a community
• Helping the Centers for Disease Control and Prevention (CDC) monitor which types and strains of flu are circulating in a given outbreak

A chest x-ray may also be done if a person is suspected to have pneumonia, a common complication of flu.

Sources:

• Influenza; University of Maryland Medical Center.
• Influenza Tests; Lab Tests Online.
• Influenza;The Merck Manual Home Edition.

Treatment for the Flu

Getting plenty of rest and fluids helps your body fight off influenza.

Most cases of the flu can be treated at home.

Treatments include rest, fluids, and the use of pain and fever reducers and/or decongestants to ease symptoms, if desired.

Getting plenty of sleep and avoiding activity when you have the flu helps your body fight off the infection.

Fluids are important to prevent dehydration.

Any type of fluid other than alcohol is fine, but it’s a good idea not to consume a lot of caffeinated beverages, which could prevent you from sleeping.

Everyday Health Flu Map: Are You At Risk?

Home Remedies for Flu

In addition to rest and fluids, the following home remedies may provide some symptom relief during a bout of flu:

• Drinking hot green, black, or herbal tea, flavored with lemon or honey, if desired
• Eating hot chicken soup
• Using a humidifier to moisten dry air
• Taking a hot shower or simply sitting in the bathroom with the shower running to create steam
• Bringing a large pan of water to a boil, taking it off the heat, then inhaling the steam by leaning over the water, with or without a towel draped over your head to create a “steam tent”
• Gargling with salt water to ease a sore throat
• Using a neti pot or similar device to rinse the nasal passages with saline
• Placing a hot compress on your forehead or nose to help with headache or sinus pain

Over-the-Counter Drugs

A number of over-the-counter (OTC) products can also relieve flu symptoms, but be sure to read the directions for use marked on the package.

Take care not to exceed the maximum daily dose for any OTC product you take:

• Acetaminophen (Tylenol)
• Aspirin, for people age 19 and older
• Ibuprofen (Advil)
• Naproxen (Aleve)
• Decongestants
• Cough suppressants or expectorants
• Throat lozenges

Pain relievers such as acetaminophen, ibuprofen, and aspirin can help to bring down a fever and reduce aches and pains.

However, taking too much acetaminophen, in particular, can be dangerous to the liver and may be unrecognized because so many cold and flu preparations contain acetaminophen.

When taking OTC cold medicines, tally up all of the acetaminophen in all of the products you’re using, and make sure you are not taking more than the FDA’s recommended maximum daily dose of 4,000 milligrams.

People under age 19 should not take aspirin during a viral illness (including the flu) due to risk of a rare but serious illness called Reye syndrome.

Herbal and Homeopathic Remedies

A number of herbs have traditionally been used to prevent or treat colds and flu. Because herbs can have side effects and can interact with some medicines, use them only under the supervision of your healthcare provider.

In general, herbal remedies are not recommended for women who are pregnant or breast-feeding.

Though there isn’t a lot of solid, clinical research into herbal remedies for flu, herbs that have shown some promise in treating flu symptom include:

• Elderberry extract may reduce flu symptoms by reducing congestion and increasing perspiration.
• Eucalyptus may relieve congestion, loosen phlegm, and soothe a sore throat.
• Peppermint, or menthol, thins mucus, relieves congestion, and loosens phlegm.
• Slippery elm may help to ease a sore throat.

Certain homeopathic remedies are also touted for easing flu symptoms, although very few studies have examined their effectiveness. Like herbal remedies, they are best used under the supervision of a healthcare provider.

Antiviral Drugs for Flu

Antiviral drugs may be used both to help prevent and to help treat flu caused by influenza A or B.

They are more commonly prescribed to people who either cannot be vaccinated against the flu or have a high risk of developing flu complications.

Those at high risk of complications include elderly people, very young children, and people with cardiovascular disease, those with chronic lung disease, diabetes, HIV infection, and other conditions that weaken the immune system.

Two antivirals are currently prescribed for flu in the United States:

• Oseltamivir (Tamiflu) is available as a liquid or a capsule. It can be used to treat flu in infants as young as two weeks and to prevent flu in children as young as one year.
• Zanamivir (Relenza) is taken via inhalation from a disk-shaped device. It is approved for treatment of the flu in children as young as 7 and for prevention in children as young as 5.

When used to treat the flu, antivirals must be taken within 48 hours of the onset of symptoms to be most effective. They typically shorten the episode of flu only by a day, but their use may prevent complications from developing.

When to Use Antibiotics for Flu

Antibiotics are only appropriate in the treatment of flu when a bacterial infection has developed in addition to the viral infection. (Antibiotics are useless against a viral infection such as influenza.)

Sources:

• Influenza; University of Maryland Medical Center.
• Acetaminophen; MedlinePlus.
• Oseltamivir (Oral Route); Mayo Clinic.
• Zanamivir (Inhalation Route); Mayo Clinic.

Types of Flu: The ABCs of Flu Viruses

There are several types of influenza viruses, including various subtypes and strains.

There are three types of flu viruses: A, B, and C. Influenza A and B cause the yearly epidemics of seasonal flu.

Influenza C causes milder illness and is not believed to cause epidemics.

Influenza A can infect a range of mammals and birds as well as humans, while types B and C infect mainly humans.

Flu viruses are constantly changing via mutations, which is why you need a new flu vaccine every year.

The flu virus changes enough each year that your immune system can’t recognize it from one year to the next. This yearly change is called antigenic drift, and it occurs in types A, B, and C.

Influenza A can also undergo a sudden, major change when two or more different flu strains infect the same cell and combine, potentially creating a new A subtype. The creation of a new subtype is called antigenic shift.

Influenza A

Influenza A viruses are divided into subtypes based on the type of two proteins — abbreviated H and N — on the surface of the virus. Those subtypes are further categorized into different strains.

Each strain of influenza A varies in the species it infects and the severity of illness it causes. Not all strains of influenza A infect humans.

Everyday Health Flu Map: Are You At Risk?

In late 2014, the Centers for Disease Control and Prevention (CDC) reported that influenza A virus subtype H3N2 was causing most cases of flu in the United States.

About 50 percent of the virus samples collected and analyzed by late November matched the H3N2 component in the 2014-2015 flu vaccine.

However, in about 50 percent of cases, the virus had “drifted,” or mutated from the H3N2 vaccine component, meaning the vaccine provided reduced protection against this new subtype.

Influenza B

Influenza B viruses are categorized into lineages and strains rather than subtypes and strains.

While influenza B typically makes fewer headlines than influenza A, it can cause severe illness.

One or two B viruses are included in the annual flu vaccine. The trivalent vaccine (containing three virus components) includes one B virus, while the quadrivalent vaccine (containing four virus components) includes two.

Influenza C

Influenza C viruses primarily cause upper respiratory tract infections in children. Although they circulate along with influenza A and B viruses during flu season, C viruses are not included in the flu vaccine.

Influenza C viruses show little genetic variability and evolve at a slower rate than types A and B.

H1N1 Influenza (Swine Flu)

H1N1 influenza is a type of influenza A. It is known as swine flu because it primarily infects pigs.

Transmission to humans is uncommon, but it can occur if humans come into contact with infected pigs or with environments contaminated by the virus. An infected human can then spread the disease to other humans.

In 2009, the World Health Organization (WHO) declared a global pandemic when rising rates of human infection with H1N1 flu were seen around the world.

The pandemic was declared over in 2010, but H1N1 is still circulating among humans as a seasonal virus and is included in the seasonal flu vaccines.

The symptoms of H1N1 flu are similar to those of seasonal flu, and can range from mild to severe.

H5N1 Influenza (Avian Flu)

H5N1 influenza is also a type of influenza A. It is carried mainly by waterfowl and can be transmitted to domestic poultry.

So-called highly pathogenic (HP) avian influenza can kill domestic birds within 48 hours.

HP avian influenza virus can also be transmitted to humans, with several hundred human cases and many human deaths reported to date. Most human cases have occurred in eastern Asia, with some cases in Eastern Europe and North Africa.

It’s believed that avian flu is passed to humans through contact with dead or live infected poultry. The virus is not transmitted through consumption of properly cooked food.

H5N1 is of high concern because it has become entrenched in poultry in several countries.

Humans likely have no immunity to it, and the resulting disease in humans is much more aggressive than seasonal influenza, with a high fatality rate.

Sources:

• Types of Influenza Viruses; CDC.
• Swine flu (H1N1 flu); Mayo Clinic.
• Avian influenza; WHO.

Getting a Flu Shot

Getting an annual flu shot is the best way to avoid getting the flu.

Many vaccines are needed only once for lifetime immunity, but the flu vaccine is needed annually. This is because the flu may be caused by a different subtype or strain of the virus each year.

Researchers are working on developing a universal flu shot, which would prevent all types of flu and do away with the need for an annual shot. But such a vaccine is not yet available.

For U.S. residents and others in the Northern Hemisphere, the best time to get a flu shot might be when it becomes available, usually in early fall. It takes about two weeks after getting the vaccine to build up immunity to the flu.

However, some doctors advise waiting to get a flu shot because your immunity to the flu decreases over the winter, a phenomenon known as “waning immunity.”

Someone who gets a flu shot in early September, for instance, will have less immunity in February and March than someone who waits until late October to get the shot.

Types of Flu Vaccines

All flu vaccines are made to protect against the three or four flu viruses that global surveillance suggests will be the most common in the coming season.

The flu vaccine is most often administered as an injection into an arm muscle or, in infants and young children, the thigh. An intradermal (“under the skin”) flu shot is also approved for people ages 18 through 64.

Everyday Health Flu Map: Are You At Risk?

Adults aged 65 or older may be advised to have a high-dose flu vaccine. The high-dose vaccine contains the same ingredients as the regular vaccine, but at a higher dose.

A study published in The New England Journal of Medicine in 2014 found that the high-dose vaccine provided better protection against the flu in older adults without any increase in side effects.

Another option is the nasal flu vaccine, which is sprayed into the nose. The nasal spray is made from weakened live virus and is approved only for healthy people ages 2 to 49 who are not pregnant.

The Centers for Disease Control and Prevention (CDC) recommends that children ages 2 to 8 receive the nasal spray, because it appears to be more effective in this population. If it isn’t available, however, they should receive a flu shot.

Flu vaccines are not 100 percent effective, meaning that some people who get vaccinated will still get the flu. However, those people are less likely to develop flu complications than people who were not vaccinated.

Current Recommendations

The CDC currently recommends that everyone aged 6 months and older get an annual flu vaccine.

The following populations are strongly advised to get an annual flu shot, either because they have a higher risk of flu complications or because they live or work with people at high risk of serious complications:

• Family members and caregivers of children younger than 6 months (who cannot get vaccinated)
• Anyone aged 6 months or older who has a chronic heart or lung condition, diabetes, kidney failure, certain blood disorders, or a weakened immune system
• Pregnant women
• Nursing home residents
• Healthcare providers
• Adults aged 50 and older

Side Effects and Other Concerns

The most common side effect of a flu shot is soreness at the injection site. Some people also develop a low-grade fever and achiness, but the flu shot cannot cause the flu because it contains no live virus.

On rare occasions, the flu shot can cause a serious allergic reaction.

Another rare but serious side effect of getting a flu shot is Guillain–Barre syndrome, a progressive nerve disorder that can cause profound weakness that lasts for weeks or longer.

The nasal flu vaccine contains a weakened live virus. Potential side effects of the nasal spray in adults include:

• Cough
• Headache
• Runny nose
• Sore throat

In children, the nasal spray may cause:

• Fever
• Headache
• Muscle aches
• Runny nose
• Vomiting
• Wheezing

Who Should Not Get a Flu Shot?

The following groups are advised against getting a flu shot:

• People who have had a severe reaction to a flu shot in the past
• People who have had Guillain–Barre syndrome
• People who currently have a condition that causes fever (other than a mild cold)

In years past, the regular flu vaccine contained a small amount of egg protein (and some still does), so anyone with an egg allergy was cautioned against getting a flu shot.

Today, however, there is a new, recombinant influenza vaccine (brand name Flublok) that contains no egg protein and is approved for people ages 18 through 49.

For people with milder allergies to eggs, the CDC guidelines advise:

• Get the recombinant influenza vaccine (RIV), if available.
• Get the regular flu shot from a provider who is knowledgeable about egg allergy and is prepared to respond to an allergic reaction.
• Remain in the provider’s office for 30 minutes for observation after receiving the flu shot.

Sources:

• Seasonal Influenza Vaccine Safety; CDC.
• DiazGranados et al. (2014). “Efficacy of high-dose versus standard-dose influenza vaccine in older adults.” The New England Journal of Medicine.
• Nasal Spray Flu Vaccine in Children 2 through 8 Years Old; CDC.

How Long Does the Flu Last?

If you’re infected with influenza, the worst symptoms usually subside in two to three days.

Flu symptoms usually start within one to four days after infection, and they may come on suddenly.

The first sign of influenza is often a fever or chills, followed by headache, sore throat, dry cough, runny nose, muscle aches, and fatigue.

As the illness progresses, a person may have warm, flushed skin, watery or bloodshot eyes, a severe cough that brings up phlegm, nasal congestion, and nausea and vomiting.

A bout of flu typically lasts for a week or two, with severe symptoms subsiding in two to three days.

However, weakness, fatigue, dry cough, and a reduced ability to exercise can linger for much longer.

Contagious Period

An adult infected with influenza may be contagious from one day before symptoms start to 5 to 7 days after becoming sick.

Children may continue to be contagious for longer than 7 days.

Staying home until your contagious period has likely passed will help you avoid passing the flu on to other people.

It will also give your body time to recover and protect you against developing flu complications or picking up another virus and getting sick again.

Everyday Health Flu Map: Are You At Risk?

Complications

When flu complications develop, a person will likely be sick for longer than a week or two, depending on the severity of the complication, how quickly a person receives treatment for it, and how well he responds to treatment.

Signs of flu complications that should prompt you to seek medical attention include the following:

• Difficulty breathing or shortness of breath
• Purple or blue discoloration of the lips
• Pain or pressure in the chest or abdomen
• High fever (i.e., temperature above 102.2 degrees)
• Sudden dizziness
• Confusion
• Severe or persistent vomiting
• Seizures
• Flu-like symptoms that improve but then return with fever and worse cough

Lingering Effects

Even in healthy people who don’t develop flu complications, the flu itself can cause symptoms that may persist for weeks, including:

• Fatigue
• Weakness
• Low appetite
• Dry cough
• Airway irritation that affects how long you can be active
• Loss of sense of smell, which in rare cases becomes permanent

Taking things slowly and not trying to rush back to exercise and other physical activities is key to rebuilding your stamina and getting back to feeling normal.

How Long Does Immunity Last?

With some viral illnesses, once you have had it or have been vaccinated for it, you’re immune for life.

With the flu, however, immunity doesn’t appear to last that long. In fact some studies suggest that immunity declines over the year following vaccination or infection.

In elderly people especially, a flu shot or infection acquired during one season does not offer much protection in subsequent seasons.

Getting a flu shot every year, therefore, is important to lowering the likelihood of getting the flu.

Sources:

• Seasonal Influenza (flu); CDC.
• Influenza; The Merck Manual Home Edition.
• Recovering From H1N1; EverydayHealth.com.
• Symptoms; Flu.gov.

Complications of the Flu

Are you in a high-risk group for influenza complications?

Approximately 200,000 Americans are hospitalized each year because of complications associated with seasonal flu, according to the Centers for Disease Control and Prevention (CDC).

The highest rate of hospitalization occurs among people aged 85 and older, but there are several groups considered at high risk of flu complications, including:

• Adults aged 65 and older
• Children younger than 5 and especially those younger than 2
• Pregnant women
• American Indians and Alaskan Natives
• Adults with pre-existing lung disease such as chronic obstructive pulmonary disease (COPD) or asthma

Annual vaccination is recommended for these groups to prevent both the flu and its complications.

Common Complications

Some of the possible complications of the flu include:

• Dehydration
• Bronchitis (inflammation of the bronchi, the airways that branch off the trachea)
• Ear infection (more common in children than in adults)
• Myocarditis (inflammation of the heart muscle)
• Myositis (muscle inflammation)
• Pericarditis (inflammation of the sac that surrounds the heart)
• Pneumonia, viral (from the flu virus itself) or bacterial
• Sinus infection (more common in children than in adults)
• Worsening of chronic medical conditions such as asthma, diabetes, or congestive heart failure

Viral Pneumonia

Pneumonia is a common complication of flu and one that can be life-threatening.

The flu virus itself can cause viral pneumonia. Symptoms include shortness of breath, chills, fever, chest pain, sweating, cough with green or bloody mucus, increased heart rate, and bluish-colored lips or nails.

Everyday Health Flu Map: Are You At Risk?

The people who are usually at highest risk of developing viral pneumonia as a complication of flu include women in the third trimester of pregnancy; elderly people, especially those who live in nursing homes; and people with chronic lung disease or cardiovascular disease.

However, certain strains of flu have appeared to affect young people more severely.

In the 1918-1919 flu epidemic, for example, many young adults died of pneumonia that experts believe was caused by the flu virus.

Bacterial Pneumonia

Several types of bacteria can cause bacterial pneumonia as a complication of flu.

Often, bacterial pneumonia starts just as a person’s flu symptoms are starting to get better.

Bacterial pneumonia causes high fevers and coughing up of greenish phlegm with shortness of breath and sometimes chest pain.

Any signs of pneumonia following the flu are reason to call your doctor.

Guillain-Barré Syndrome

Guillain-Barré syndrome can be a complication of the flu or of the flu shot. It’s a form of peripheral neuropathy that is believed to result from an autoimmune reaction.

Weakness is the predominant symptom of Guillain-Barré, usually beginning in both legs and progressing up the body to the arms.

Guillain-Barré syndrome can progress rapidly and is considered a medical emergency, often requiring hospitalization.

The nerve damage stops progressing within eight weeks, and most people improve, although some can have residual weakness.

Reye Syndrome

Reye syndrome is a rare condition that can develop when a child or teenager takes aspirin during a viral illness such as the flu. Reye syndrome causes inflammation and swelling of the brain and degeneration of the liver.

Symptoms include severe vomiting, lethargy, irritability, and restlessness.

In children younger than 2, diarrhea and hyperventilation may be the first signs of Reye syndrome.

The severity of illness can vary widely, but any suspected case of Reye syndrome should be treated as a medical emergency.

When to Seek Medical Help

According to Flu.gov, a website managed by the Department of Health & Human Services, if you or someone in your household experiences any of the following symptoms, seek medical help immediately:

• Difficulty breathing or shortness of breath
• Purple or blue discoloration of the lips
• Pain or pressure in the chest or abdomen
• Sudden dizziness
• Confusion
• Severe or persistent vomiting
• Seizures
• Flu-like symptoms that improve but then return with fever and worse cough

Sources:

• Seasonal Influenza-Associated Hospitalizations in the United States; CDC.
• Influenza; The Merck Manual Home Edition.
• Symptoms; Flu.gov.

When Is Flu Season?

The definition of flu season depends on where you are.

Flu season happens at different times of the year in different parts of the world.

Flu season typically occurs in the fall and winter. The peak of flu season — the month with the most confirmed cases — usually takes place between December and February in the northern hemisphere, but it has been both earlier and later.

Seasonal flu outbreaks can start as early as October and last as late as May.

In temperate areas of the southern hemisphere, flu season runs from April through September.

In the tropics, flu can be spread year-round but tends to peak during the rainy season and/or during times of high humidity.

Everyday Health Flu Map: Are You At Risk?

Tracking the Flu

Because flu season happens at different times in different areas of the world, health experts can track which viruses are causing the most illness in one part of the globe before they reach another part.

For example, it’s often possible to make predictions about flu season in the northern hemisphere based on what has happened in the southern hemisphere.

Currently, a network of scientists assembled by the World Health Organization (WHO) called the Global Influenza Surveillance and Response System (GISRS) monitors flu outbreaks around the world and makes recommendations regarding which virus strains to include in the seasonal flu vaccine.

The GISRS posts data and reports on FluNet, a public website that anyone can access.

In the United States, the Food and Drug Administration (FDA), with input from the Centers for Disease Control and Prevention (CDC), makes the final decision on the composition of the flu vaccine for the U.S. market.

An FDA advisory committee meets annually in February to pick which viruses to include in the vaccine for the flu season to start the following fall.

When to Get a Flu Shot

It takes about two weeks to develop immunity to the flu after getting the flu shot, so ideally, people should get vaccinated well before they are likely to be exposed to the flu.

For the 2014-2015 flu season, distribution of flu vaccine to pharmacies, medical offices, and other sites that administer flu shots began in August 2014.

But just as flu season varies from year to year, the date at which flu shots become available can vary from one year to the next as well.

In the United States, flu vaccines are made by private pharmaceutical companies, and it can take four to six months for them to manufacture large quantities of the vaccine. Any problems encountered during production can lead to shortages or delays in supply.

Travel and the Flu

The CDC recommends that Americans traveling to parts of the world where there is flu activity get vaccinated at least two weeks before they go.

The vaccine formulated for the northern hemisphere will usually prevent against the major types of flu circulating in other parts of the world.

People at high risk of developing flu complications may wish to speak to their doctor about taking flu antiviral drugs with them on their trip. When taken within 48 hours of the onset of symptoms, these drugs can shorten the length of the illness and help to prevent flu complications.

The CDC does not recommend getting revaccinated for summer travel, so a person who was vaccinated for the northern hemisphere flu season in the fall does not need another flu shot prior to travel to the southern hemisphere the following summer.

Sources:

• Seasonal Influenza (flu); CDC.
• FluNet; WHO.
• Influenza Prevention: Information for Travelers; CDC.

Historic Flu Outbreaks

Flu pandemics have occurred throughout history and will almost certainly happen again.

Pandemics are global outbreaks of disease. A flu pandemic typically occurs when a new influenza A virus emerges that can infect humans.

If humans have little or no immunity to the new virus, the flu spreads rapidly and widely from person to person.

Flu pandemics often occur in waves, with two or more distinct peaks in numbers of infections and deaths, separated by months or even years.

Russian Flu (1889-1890)

The 1889 Russian flu (also called the Asiatic flu) was the largest flu epidemic to affect Europe in the 19th century. The epidemic peaked first in St. Petersburg, Russia, and spread in waves throughout the northern hemisphere.

Even in the absence of air travel, the flu traveled across the globe in a matter of months. It ultimately killed an estimated 1 million people worldwide.

Spanish Flu (1918-1919)

The flu pandemic that started in 1918 is the deadliest in recorded history. Estimates of the number of people killed worldwide range from 50 to 100 million.

According to Flu.gov, there were nearly 675,000 deaths in the United States.

The Spanish Flu was remarkable for making people very sick very rapidly. Some people reportedly woke up in the morning feeling fine and were dead by the end of the day.

Everyday Health Flu Map: Are You At Risk?

Many of those who died in the pandemic died of secondary bacterial infections, such as bacterial pneumonia.

The Spanish flu is believed to have been caused by an avian flu H1N1 virus that evolved to a form that could easily infect and spread among humans.

Asian Flu (1957-1958)

A new influenza A virus subtype H2N2 was identified in China in 1956. The new virus originated from a mutated avian flu virus that combined with a human flu strain. It spread within East Asia in early 1957 and arrived in the United States during the summer of 1957.

At the outset of the pandemic, school children, young adults, and pregnant women were the hardest hit. By the end, however, the highest rate of deaths was among the elderly.

A vaccine for the H2N2 virus introduced in 1957 slowed the pandemic, which ultimately killed about 69,800 people in the United States, according to Flu.gov, and 1 to 4 million worldwide.

Hong Kong Flu (1968-1969)

In early 1968 a new influenza A virus subtype H3N2 emerged in Hong Kong. This flu was highly contagious and spread across the globe rapidly.

However, it caused comparatively fewer deaths than previous pandemics, with an estimated total of 33,800 in the United States, according to Flu.gov, and 1 million worldwide.

Because the Hong Kong flu bore some similarity to the Asian flu of 1957, people affected by the earlier flu may have had some immunity to the Hong Kong flu, limiting the number of fatalities.

Russian Flu (1977-1978)

A new strain of H1N1 appeared in 1977 that spread worldwide but had a low fatality rate. It mainly affected children and young adults born after 1950.

Those born before 1950 had some immunity to the new strain because it was similar to strains that had circulated in the years before 1950.

2009 H1N1 Pandemic

Yet another new strain of H1N1 emerged in North America in 2009 and quickly spread throughout the world. Although the virus was initially called “swine flu,” it was found to contain genetic material from four different flu viruses — two swine, one avian, and one human. The new virus had never before been seen in humans or animals.

The 2009 H1N1 flu was notable for primarily affecting children and younger adults. According to CDC statistics, about 90 percent of hospitalizations and 87 percent of deaths in the United States were in people younger than 65.

The CDC has estimated that between 43 and 88 million people in the United States were infected with the 2009 H1N1 flu, and between 8,870 and 18,300 died.

The CDC estimate for deaths worldwide is between 151,700 and 575,400.

The same strain of H1N1 that caused the 2009 pandemic has continued to circulate among humans as a seasonal flu virus. It is included in the annual flu vaccine.

Looking to the Future

Public health officials have expressed concern that two types of avian influenza, H5N1 and H7N9, have the potential to cause future pandemics.

Thus far, these viruses have infected only a limited number of people, and they are not easily transmitted from one human to another.

But if either virus were to change slightly so that the infection could travel more easily between people, the result could be virulent, widespread disease.

Sources:

• About Pandemics; Flu.gov.
• CDC Resources for Pandemic Flu; CDC.
• The 2009 H1N1 Pandemic; CDC.

Flu News

Source: http://www.everydayhealth.com