Schizophyte

Schizophytes were a form of bacteria which were the smallest known living organisms. Under normal circumstances, they would exclusively be found in plant life rather than in humans.

However, as the Fourth Doctor, Derek Moberley and John Stevenson discovered when examining a blood sample taken from Charles Winlett, individuals who were in the process of transforming into a Krynoid would have schizophytes develop within their bloodstream in the place of blood platelets (with the Doctor remarking that Charles's blood was "turning into vegetable soup"). (TV: The Seeds of Doom)