It could be some latent 'junk' DNA being triggered by something (yes I got the idea from a Dr Who episode)...

Are these infected people or magical walking corpses? 'Zombies' used to be people who'd been drugged, buried, and used for cheap labor in a sugar mill; nowadays they're rotting corpses that somehow walk and eat. Even "Dr Despicable" can't wrap his head around it (from Dr Despicable's Chamber Of Cinema podcast)

Have the "We're Alive" zombies survived fatal injuries? Do their severed heads still bite? The tower people NEED to study one up close! Especially now that they have a Doctor on board.... I don't think they're walking corpses OR voodoo zombies; they seem like mutants of some kind, people who have changed but not died; their higher minds are gone, they're savage and instinctual.