Injected Pathogen
Some creatures can inject dangerous biological agents into a starship. When calculating its boarding statistics, the pathogen’s tier equals the creature’s tier, and the pathogen’s complement modifier is +0. In addition to fending the pathogen off as though it were a boarding party, a defending officer can use environmental controls to weaken the pathogen as an open crew action with a Life Science check (DC = 10 + 1-1/2 × the creature’s tier). If successful, all pathogen boarding parties take a –4 penalty to their next boarding attack before the end of the next round.
Source
Starship Operations Manual pg. 40