Reye syndrome or Reye's syndrome is an extremely rare rapidly progressive encephalopathy which usually begins shortly after recovery from an acute viral illness, especially influenza and varicella (chickenpox). It is a potentially fatal syndrome that has numerous detrimental effects on many organs, especially the brain and liver, as well as causing hyperammonemia (elevated blood ammonia level) and low blood sugar. The classic features are a rash, vomiting, and liver damage. The exact cause is unknown and, while it has been associated with aspirin consumption by children with viral illness, it also occurs in the absence of aspirin use. The disease causes fatty liver with minimal inflammation and cerebral edema (swelling of the brain). The liver may become slightly enlarged and firm, and there is a change in the appearance of the kidneys. Jaundice is not usually present.