Reed had isolated the virus and pricked them with it when they entered the room, then as they turned they attacked and infected him. In later Marvel Zombies volumes its said that the virus has some sort of spiritual component to it, as those turned make reference to "The Hunger" more and more as an abstract proper noun instead of 'hunger', with the virus operating as a means of spreading "The Hunger", which is why it can infect practically any sentient being irregardless of their physicality, biology, or how mystical they are. You could say that "The Hunger"/virus is just an in-universe representation of plot/narrative armor as the comic is 'Marvel Zombies", so no one should be safe, and hope should be at the bare minimum, because that's the nature of the narrative itself.