crookedman: That's a Skrasher they incubate in peeps bodies and come bursting out, basically they're aliens from Alien. The eggs can be removed with a Z-Ray scanner in the Sick Bay, but that's still pretty risky as there is something like a 200 second incubation period. The cat like Memaus are the larval form they eat garbage until they become infectious. Sometimes the peeps will be carriers and bring them on board unwittingly.

If you get a medical emergency message from a Nostro class ship they will be infected as well.