From wiki
at some point, Jenny learns she is suffering from an unknown virus and sends a letter to Forrest to come see her. She tells Forrest about their son, and her illness, apologizing if her problems in her past ever hurt him. Forrest invites her and Forrest Jr. to come stay with him. She and Forrest get married soon after. They spend a few happy months together as a family before she finally dies on March 22, 1982 Forrest has her buried next to the tree where they played as children and adults and has her father's house demolished. Though Forrest misses her terribly, he becomes a caring father to Forrest Jr.
Was also interesting to note that she did not die in the first book. Didn't happen until the sequel.