Goku contracts a heart virus in the 766th year of his life. However, the symptoms of the disease start to appear when Goku fights Android 17 and 18.
It is unknown when Future Goku began experiencing symptoms
Goku contracts this virus after his battle with the Androids. He then goes to Capsule Corporation where he is treated by the family doctor, Dr. Brief. Future Trunks attempts to fight Goku but is unable to do so until Goku loses consciousness from the virus.
Goku ends up on the bed and spends two weeks in bed without moving. Chi-Chi begins getting worried about him so she brings a doctor (who looks like Majin Buu) over to check Goku for a possible operation, but Future Trunks stops her and says that Goku will be fine after resting in bed for some time.
It is unknown when Future Goku contracted this virus, but it must have been after his battle with the Androids. “Bills” was in Goku’s body, and so was Majin Buu.
Goku has a strange fever, which is either a symptom of the heart disease or a separate illness. He also experiences unusual changes in personality: becoming cold and ruthless, similar to his evil potential shown in “The Evil of Men”, and developing an insatiable desire to fight stronger opponents. When Future Trunks and Vegeta fail to defeat Goku as he transforms into a Super Saiyan, they join forces and manage to seal him inside the Sealed Ball.
Future Trunks arrives from the future and gives Goku the antidote to the Heart Virus . However, Goku ends up in the hospital.
Goku takes his Time Machine to the past and fights a giant ape-like creature who is revealed to be one of the remaining chimps from the zoo that previously exploded. The fight turns into a fierce battle with Goku emerging victorious. He goes back to the future and gives Future Bulma a letter for Bulma to give to Android 18 and 17, telling them that their father has been cured and he wants them to live on Earth with him and Future Trunks in peace rather than destroying it like they originally intended.