microzyme

from The Century Dictionary.

• noun One of a class of extremely small living creatures, existing in the atmosphere, and furnishing the basis on which certain epizoötic, epidemic, and other zymotic diseases are dependent for their existence; a zymotic microbe.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Biol.), obsolete A microörganism which is supposed to act like a ferment in causing or propagating certain infectious or contagious diseases; a pathogenic bacterial organism.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology, dated A microorganism supposed to act like a ferment in causing or propagating certain infectious or contagious diseases; a pathogenic bacterial organism.