A sub-species from a planet light years away from Earth who can change shape into a wolfish creature depending on the phase of the moon. On their planet, that's every night cause their moon's always in the full, but on Earth that only happens about once a month, hence the werewolf myth. Their cells can infect other lifeforms through a bite or a scratch and cultivate themselves over years and years until the host becomes one of them too. 'Course, the host's mind usually goes years before that because the transfer of cells also transfers their mind as well, so it's like a complete body takeover.