EPSTEIN-BARR. BtVS Intertext/Allusion.
In a scene set in The Bronze in "Welcome to the Hellmouth" we overhear Cordelia say the following:
My mom doesn't even get out of bed anymore. And the doctor says it's Epstein-Barr. I'm like, pleeease! It's chronic hepatitis, or at least chronic fatigue syndrome. I mean, nobody cool has Epstein-Barr anymore.
The website for the Center for Disease Control offers the following account of Epstein-Barr:
Epstein-Barr virus, frequently referred to as EBV, is a member of the herpesvirus family and one of the most common human viruses. The virus occurs worldwide, and most people become infected with EBV sometime during their lives. In the United States, as many as 95% of adults between 35 and 40 years of age have been infected. Infants become susceptible to EBV as soon as maternal antibody protection (present at birth) disappears. Many children become infected with EBV, and these infections usually cause no symptoms or are indistinguishable from the other mild, brief illnesses of childhood. In the United States and in other developed countries, many persons are not infected with EBV in their childhood years. When infection with EBV occurs during adolescence or young adulthood, it causes infectious mononucleosis 35% to 50% of the time.
More information about Epstein-Barr can be found here: http://www.cdc.gov/ncidod/diseases/ebv.htm.