Tribbles, though originally from some planet called Iota Geminorum IV, can live just about everywhere, even the cold vacuum of space. They are especially fond of living on star ships, airplanes, and in Captain Kirk's sock drawer. Some might say they only reproduce when conditions are favorable, but this is a lie, and these people obviously trust Tribbles way too much. Just about any condition is favorable for a Tribble to reproduce in, thus taking one Tribble anywhere is damning everyone within a 10-mile radius.