Delirium refers to a state of severe confusion and rapid changes in brain function that comes on suddenly. It can be associated with hallucinations and hyperactivity, and the sufferer is inaccessible to normal contact. Delirium can occur due to a wide variety of causes, including head injury, drug use or withdrawal, poisonings, brain tumors, infections, and metabolic disturbances. A urinary tract infection or pneumonia, for example, may lead to delirium in older people. The prognosis of delirium depends upon the underlying cause of the condition. Delirium can also occur as a result of mental illness. Symptoms can include changes in alertness or level of consciousness, confusion, drowsiness, incontinence, delusions, changes in speech, emotional changes, decreases in memory, concentration problems, disorientation, disorganized thinking, and changes in perception. Delirium is sometimes mistaken for depression or dementia.