It may have ended the lives of 25 million people in a single year in the sixth century, but the plague—caused by the bacterium Yersinia pestis—is exceedingly rare in the US, infecting about seven people a year, per the CDC. But in what officials tell ABC News is the first such case on record in the States, a human got the disease from a dog; the small outbreak ultimately involved four people, all of whom recovered, reports NBC News. The outbreak of pneumonic plague—the most serious of the three forms, and the only one that humans can transmit to each other—was in rural Colorado. That fits with one doctor's statement to NBC that the "plague is virtually always confined in this day and age to rural regions in the West." That's thanks in large part to populations of prairie dogs, who typically spread the disease through fleas.
In this case, an American pit bull terrier developed a fever and jaw rigidity last summer and had to be euthanized the very next day. The dog's owner went to the hospital four days later with a fever and bloody cough. A Colorado health official tells ABC the source is a surprise. "Primarily ... dogs don't get sick at all or they get a minor illness." More often, cats play with prairie dogs, pick up infected fleas, and spread the bacteria to people when they cough infected blood or mucus. Another surprise, per NBC: Human-to-human transmission may have occurred for the first time in 90 years. In addition to the owner and two veterinary workers, a friend of the owner was infected. While she did interact with the dog, the timeline of her infection suggests she may have gotten it from the owner. (A study may have identified the true root of the Black Death.)