>>537842989
Gohan says "He was healthy the whole time so he never took the medicine", so I assume he somehow took longer to get sick at all.

I don't subscribe to the theory "He got the heart virus in Yardrat", Trunks talks like it was some incurable disease in current times, but later on there was one, so it might've been just a particularly nasty earth virus.

So I think he just, got infected with it later than it normally would. Could've been from something as simple as eating something from the fridge that had the virus months after his future counterpart did. Manga doesn't imply why it happened earlier (Trunks did theorize the creature that came out of the other time machine could've been the cause, but that turned out to not be the case), so I can't think much why that happened.