Zaris of the Argomeld
From Tardis Wiki, the free Doctor Who reference
The Zaris of the Argomeld was a race of beings from another universe. One Zaris infected Myia as a host.
The Zaris would jump from host to host in order to grow from macrobial size to a larger size, until returning back to his home universe, and would destroy the surrounding area in the process. (PROSE: The Pythagoras Problem)