The world of Dragon Ball is filled with incredible and fantastical beings and places. There are alien overlords, super-advanced cyborgs and magical orbs that can easily raise the dead. At the center of it all is Son Goku, a Saiyan with seemingly ever-growing power. No matter how powerful or godlike an opponent may be, Goku has always risen to the challenge and matched their strength.
In this world of near-omnipotent figures, it can be easy to forget that Goku is still mortal. No matter how powerful he gets, he is not immortal. One battle in Dragon Ball Z reminded both the protagonist and fans alike of that reality. During the "Android Saga," Goku nearly lost his life -- not to a terrifying opponent, but to a simple heart virus.
At the very beginning of the "Android Saga," Future Trunks travels back in time to warn the Z Fighters of the upcoming threat to Earth. In his timeline, two androids built by Dr. Gero appeared and killed all the Z Fighters. Goku was unable to stop him because he'd already died of a heart virus. To prevent that from happening in the current timeline, Future Trunks warned Goku of the virus and provided him with special medicine.
Unfortunately, Goku neglects to take his medicine while waiting for the androids to appear. Because of this, the heart virus manifested while he was fighting Android 19. Vegeta ultimately saved him from being killed by the androids, but Goku is forced to sit on the sidelines for a large portion of the "Android" and "Imperfect Cell" sagas. He can only writhe in pain while his friends struggle to protect Earth from this new threat.
Luckily, Goku gets the medicine he needs in time and manages to overcome the heart virus, eventually recovering and achieving new levels of strength as he always does. Although it was only a brief moment, Goku's battle with a heart disease has a lot of interesting implications for his character that could be explored in the future.
It is incredibly rare for a shounen anime character to die of natural causes. Usually they meet their end to a far stronger opponent, or sacrifice themselves to protect their loved ones. Goku himself has died in these ways multiple times throughout Dragon Ball. The fact that he died in one timeline from heart disease and almost died in the same way in another proves that he's not invincible.
Despite being an alien from another world with incredible powers, Goku is still mortal and vulnerable to all the same diseases and health issues that plague normal humans. Although he survived the heart virus, who's to say that it doesn't return, or that he contracts some other potentially fatal disease?
Aging is another issue that could affect Goku. Saiyans seem to have a similar lifespan to humans and although they remain at their physical peak longer than humans, they can die of old age. What would happen if Goku became elderly and could no longer fight as he did in his prime? How would this Saiyan, who loves fighting so much, handle that reality? His near-death experience with the heart virus is a stark reminder that there are some problems strength can't solve.
The heart virus also revealed an interesting aspect of Goku's character. Even though he was directly warned about the disease and knew it could kill him, he still didn't take his medicine. This could easily be attributed to his lack of intelligence or maturity, but it's also deeper than that. Goku's interest lies solely in fighting and martial arts. Instead of worrying about a disease that could be his death, he spent three years doing nothing but training.
If it doesn't have anything to do with fighting or training, Goku has no patience for it. This mindset has led to a lot of issues for him and the people around him. After defeating Frieza on Namek, Goku willingly chooses to stay on an alien planet to learn a new technique instead of being teleported to his friends and family. Decisions like this have caused him to be absent for much of his children's lives.
Goku's lack of education, maturity and interest in other aspects of life have left him ill-suited for the everyday world. He isn't the most attentive father or husband, jobs hold no value for him, and the heart virus proves he isn't capable of caring for his own health. Despite all the power-ups and transformations, the famous Dragon Ball protagonist is still a mortal who has to deal with everyday problems. Maybe Goku should direct more of his attention toward the mortal side of his life while he still can.