Along with the scientists' bodies the group finds a dog, which attacks Mulder and Bear. Scully notices black nodules on its skin and suspects that it may be infected with bubonic plague; she also notices a rash on its neck and movement beneath its skin.
The discovery of a dog, which attacks Mulder and Bear, leads to the suspicion that it may be infected with bubonic plague. Scully's observations of the dog's skin, rash, and internal movements suggest this possibility.