It wasn’t Hep C. though. People saying it was are talking about the book, which is different.
In 2019, Forrest Gump screenwriter Eric Roth confirmed that the illness Jenny died from was late-stage HIV. During an interview (via Yahoo Entertainment) about the film’s 25th anniversary, Roth discussed the details of a sequel that was canceled after the 9/11 terrorist attacks in 2001. He reveals that the Forrest Gump sequel was actually going to open with the revelation that Forrest Jr. had late-stage HIV, the result of acquiring the disease from his mother, Jenny.