Protocystian spores are a hazardous lifeform that can 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. (ENT: "Broken Bow")
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. (ENT: "Singularity")