Humans are scary. It's a view held by all sentient creatures throughout the galaxy, one formed after first contact with Earth. When a plague breaks out and turns them feral (think Zombies), a gracious and kind alien civilization takes it upon itself to cure those affected. At least, that's the history our janitorial heroes have been told.

When a bioweapon is released and the cured revert, only a handful of the crew manages to keep their health and their wits. Mostly.