Four people came down with the pneumonic plague in rural Colorado after contact with an infected pit bull, reported the Centers for Disease Control and Prevention Friday. 

The report says that one of the cases may have been transmitted from human to human, which would be the first time that's happened since 1924. 

Colorado's Tri-County Health Department helped investigate the case, and the group's executive director explained more to NPR about how the dog's owner and three others got sick:

The pit bull may have picked up the plague from prairie dogs in his rural neighborhood. Those rodents harbor plague bacteria. Turns out, the dog spread pneumonic plague to two people at the vet clinic where he was treated before the dog succumbed to the disease. A woman identified only as Patient D also fell ill after she had extended contact with the dog as well as the man.