Jenny Died from Hepatitis C
Towards the end of Jenny's life, she tells Forrest, "I have some virus, and the doctors don't know what it is, and there isn't anything they can do about it." Viewers know through context clues that Jenny was a drug addict, and, though the movie never states what she was sick with, Hepatitis C was her confirmed cause of death in Winston Groom's sequel "Gump & Co." Hepatitis C was unknown in the medical community in the 1970s and only gained real relevance in the 1980s.