“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.”
Source: US NIH
It was first described in 1963 by Dr, Douglas Reye, and has been found to be associated (in around 90% of cases) with the use of aspirin which had been prescribed to control the fever of the previous infection. Although uncommon, It is a serious complication, with a 30% mortality rate. Those who recover often have debilitating long-term health problems.
The cause (other than the fact it's associated with aspirin) is unknown.
“Much of the research on RS focuses on answering fundamental questions about the disorder such as how problems in the body's metabolism may trigger the nervous system damage characteristic of RS and what role aspirin plays in this life-threatening disorder.”