Jenny dies without us learning anything more about her disease, but it has to be AIDS, right? She had been a drug addict and a prostitute, and she dies in 1982, the same year the media first reported on it. Surely this is an extremely dark twist on Gump's 20th-century retrospective? The movie's fans widely assume it's AIDS, discussing it as a given, and the University of Pennsylvania has an essay discussing how Jenny's poor life choices ended with her contracting AIDS. Even Family Guy did a bit on how Forrest is too dumb to grasp the implications of sex with an HIV-positive woman.
But the movie never actually says "AIDS." Jenny drops dead of Plot Convenience Syndrome, and viewers rushed to assumptions based on the fact that she slept around. The sequel to the original novel, Gump And Co., clarifies that it was in fact meant to be hepatitis C. That fits the timeline perfectly. Hep C wasn't identified until 1989, but "non-A, non-B hepatitis" was acknowledged as a mysterious new disease in 1981, the same year Jenny reveals her condition to Forrest. Granted, it did have a name, but it's not unreasonable to assume that Jenny left as many multi-syllabic words as possible out of her explanation to Forrest. At least you don't have to worry about Forrest and his son suffering slow, painful deaths after the movie anymore.