Spider-man explained it. Anyone with any sort of accelerated healing should have been able to fend off the virus longer than others, but in a immunological war of attrition, the virus wins. Viruses evolve and develop ways to beat immune systems. for example, some viruses rely on creating proteins that mimic target specific components of the immune system, or prevent immune system recognition of virally infected cells.
As for the Hulk, the virus travelled through space, meaning it was likely constantly exposed to Cosmic Radiation, which has been noted to cause cellular mutations, such as the possibility of resistance to other forms of radiations.
The virus was also sentient and was targeting those with powers, as they would be the ones most likely to cross dimensional barriers, so it could spread it's gospel.
There's only one character who was known to be completely immune to the virus, and that was the Conquistador in 616, and that was possibly because he had bathed in the Fountain of Youth. Still got devoured by the zombies though. (Marvel Zombies 3)
In the end, it was a moot point. They wanted something that would make heroes into nightmarish monsters and the best way they could have done that was a virus that was 100% unstoppable. Hell, its why Uatu forced it into a timeloop to stop it from spreading out beyond two destroyed universes.
