Reye's syndrome (RS) is primarily a children's disease, although it can occur at any age. It affects all organs of the body but is most harmful to the brain and the liver--causing an acute increase of pressure within the brain and, often, massive accumulations of fat in the liver and other organs. RS is defined as a two-phase illness because it generally occurs in conjunction with a previous viral infection, such as the flu or chicken pox. The disorder commonly occurs during recovery from a viral infection, although it can also develop 3 to 5 days after the onset of the viral illness. RS is often misdiagnosed as encephalitis, meningitis, diabetes, drug overdose, poisoning, sudden infant death syndrome, or psychiatric illness. Symptoms of RS include persistent or recurrent vomiting, listlessness, personality changes such as irritability or combativeness, disorientation or confusion, delirium, convulsions, and loss of consciousness. If these symptoms are present during or soon after a viral illness, medical attention should be sought immediately. The symptoms of RS in infants do not follow a typical pattern; for example, vomiting does not always occur. The cause of RS remains a mystery. However studies have shown that using aspirin or salicylate-containing medications to treat viral illnesses increases the risk of developing RS.