Dragon Ball's Son Goku is one of anime's mightiest, most prolific protagonists. Throughout his career, he's battled monsters, armies and a powerful cadre of villains, each one more daunting and powerful than the last. And -- true to shonen hero form -- Goku has risen to and defeated each challenge. However, one of Goku's closest calls wasn't against the red Ribbon Army or a god of destruction. Instead, it arrived in the form of a debilitating heart virus.
At the close of the "Future Trunks Saga," Future Trunks explained to Goku that he would eventually contract a virus -- one that specifically attacked the heart. In Future Trunks' time, a cure for the virus hadn't yet been developed, and the Goku of his timeline consequently died. However, a cure was eventually developed, which Future Trunks delivered to Goku in the hope of preventing his and the rest of the Z Fighters' deaths at the hands of Dr. Gero's maniacal androids.
True to Future Trunks' warning, Son Goku eventually began to show symptoms of the heart virus while fighting Android 19. Between the excruciating pain and fatigue caused by the heart virus and Android 19's energy absorption abilities, Goku only escaped death thanks to Vegeta's timely intervention.
But how is it that Goku -- a Super Saiyan at the peak of his abilities who hadn't been grievously sick at any other point in his life -- succumbed so quickly to a virus? Could he have contracted some kind of superbug during his time in space? Was the heart virus a consequence of Goku biting Frieza's tail during a particularly desperate point in the battle? Or could it be that the heart virus was created by the Red Ribbon Army's despotic genius Dr. Gero?
While it may seem like a stretch to attribute the heart virus to Dr. Gero, it isn't a completely far-fetched theory. Gero was well versed in robotics and biology. He built a number of androids with the Red Ribbon Army, at least two of which -- Androids 17 and 18 -- were once living human beings. He also developed a hyper-intelligent, ultra-miniature surveillance system that not only monitored Goku and the other Z Fighters but also sampled the genetic material of the strongest warriors to ever set foot on Earth (those being Goku, Piccolo, Vegeta, Frieza and King Cold). The ultimate culmination of his efforts was none other than the biomechanical terror Cell.
Additionally, Dr. Gero harbored a deep-seated hatred not only for Goku -- who trounced the Red Ribbon Army as a mere teenager -- but for the entire human race. Before the Dragon Team arrived to challenge them, Gero (as the suped-up Android 20) and Android 19 went on a killing spree, wreacking havoc upon bystanders who happened to cross their path. Gero also unleashed his laser-eye attack when Goku asked to move the battle away from innocent people, demolishing the city and illustrating his contempt for human life.
Given his villainous track record and incredible genius, creating a virus that's powerful enough to bring down even Goku's Super Saiyan heart seems well within Dr. Gero's purview. However, if the heart virus was indeed his own creation, wouldn't it have been in his best interests to avoid showing himself until he was certain the virus was functioning properly? Why risk fighting and being destroyed by an enemy when a targeted, stealthy assault is already underway?
What's more, if the virus was in fact designed by Gero and strong enough to incapacitate Goku, it seems likely that Vegeta would be an equally susceptible target. Yet Dr. Gero was visibly startled by Vegeta's appearance and Super Saiyan transformation -- hardly the behavior of someone who's just released a microscopic secret weapon.
While a definitive origin behind Goku's heart virus remains a mystery, it isn't unreasonable to suspect Dr. Gero of having a hand in it. True, there's more than a swath of evidence to suggest otherwise, but few antagonists in Dragon Ball Z possess Gero's unique combination of intellect, ruthlessness and foresight. After all, Dr. Gero programmed his supercomputer to complete and give birth to Cell after his own death. Thus, out of an abundance of caution, it seems logical that Gero would create and unleash a virus that could -- should he prematurely meet his own end -- fulfill his ultimate plan of defeating Goku and decimating humanity.