Topsiders' immune systems have degraded after decades in the suits suits
If the flash backs/ditto projections can be trusted, it took between several minutes and several hours for humans to transform into monsters/anthro's after exposure, but the topsiders seem to transform in seconds after the suit breaches.- While the virus is revealed to be interdimensional merging with alternate-universe counterparts, it turns out the material in the suits is decidedly not normal, so they could have compromised immune systems either way.
The virus is sentient and making choices
Transforming people differently if they are awake, asleep or somewhere in between and giving a healing power to anthro's sounds like too much for a virus to achieve on its own. But then again, werewolves and vampires in modern fiction are often created by viruses.- Jossed, it's anti-choice; the boundaries between dimensions are shattered in Endtown's world, which causes unconscious humans to merge with their alternate-universe cartoon selves. If both counterparts are conscious enough to make a choice, their minds will subconsciously attack each other, effectively killing each other and mutating the remains.
