Protocystian spore

Protocystian spores were a hazardous lifeform that could be found on the planet Rigel X. Commander Charles Tucker and Subcommander T'Pol were exposed to these spores in April 2151, necessitating a stay in the decontamination chamber. 

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