Protocystian spores were a hazardous lifeform that could be found on the planet Rigel X.
In April 2151, Commander Charles Tucker and Subcommander T'Pol were exposed to these spores, necessitating a stay in the Enterprise NX-01's decontamination chamber. (ENT: "Broken Bow")
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. (ENT: "Singularity")