Fairly sure it's heavily implied sheShe had Hep C.
dies from HIV/AIDS.
As I say, it was mostly tongue in cheek, I do realistically understand she should be a sympathetic character but hard to with the way she's written and the emotional hurt she also gives to Forrest.I don't think Jenny sleeps with him after she's diagnosed with the (then unknown) HIV.
Jenny considers herself unworthy of any love because of the abuse she suffered as a child at the hands of her father. She knows she's broken and keeps leaving Forest because she doesn't want to drag him down to what she considers her level. It's only when she has a child that she offers and gains unconditional love, and has someone else to live for that she understands she's more than just a broken child grown into a broken woman.
It's funny how many people see this film (myself included) really dislike Jenny for her comings and goings, her inability to commit, etc, but when it's explained from her point of view, she's really a character in need of help and sympathy to overcome her trauma and damage, which Forest eventually doesby loving her, giving her a child, and eventually marrying her.