Protocystian spore
Protocystian spores were a hazardous lifeform that could be found on the planet Rigel X. 

In April 2151, Commander Charles Tucker and Sub-Commander T'Pol were exposed to these spores, necessitating a stay in 's decontamination chamber. ()

In 2152, when Doctor Phlox insisted on examining Travis Mayweather, he reasoned this with the possibility, that he could carry protocystian spores and infect the entire crew. ()

de: Protozystianische Spore
Microorganisms