Dog Infects Humans With Plague for First Time in US Four people were infected with the dangerous disease last summer.

 -- A plague-infected dog spread the dangerous disease to four Colorado residents, according to a new report from the U.S. Centers for Disease Control and Prevention.

Health officials told ABC News that this the first report of a dog infecting a human with the plague in the U.S.

The dog, a 2-year-old American pit bull terrier, became sick last summer with a fever and jaw rigidity, among other symptoms. The dog's health declined so quickly that it was euthanized the following day at a local vet's office, health officials said.

Four days later, the dog's owner entered the hospital with a fever and a bloody cough that became worse over the next few hours, but an initial blood culture was misidentified, according to the CDC report.

As the patient's symptoms grew worse, the test was redone and he was found to have been infected with pnumonic plague, according to the CDC report. The remains of the dog were also tested and were found to be positive for the plague bacteria.

"Frankly one of the biggest surprises of this outbreak is the source," said John Douglas, of Tri-County Health Department in Colorado and one of the study authors. "Primarily ... dogs don’t get sick at all or they get a minor illness," after being infected with the plague.

Janine Runfola, of the Tri-County Health Department in Colorado and lead author of the report, explained that cats are more likely to infect humans with the disease than dogs because they exhibit more symptoms.

"For pneumonic plague a more likely scenario would be you have a cat [play] with prairie dogs and infected fleas get on the cat," Runfola said. "The cat gets sick and sneezes and coughs on its owner."

The dog's owner remained hospitalized for 23 days as he recovered from the potentially deadly disease, the report said. In addition to the owner, a close contact of the owner and two veterinary employees who treated the dog or handled its body also became infected with the plague. All three were successfully treated with medication after exhibiting symptoms.

The plague is caused by the bacteria Yersinia pestis, and can infect the body in different ways. For example a flea bite can lead to infection of the glands, which is called bubonic plague -- notorious for the epidemics it spawned during the Middle Ages in Europe. Because this plague was spread from dog to owner through coughing, it developed into pneumonic plague, according to Douglas.

The plague is known to be endemic to prairie dogs in the American Southwest, which can then lead to isolated outbreaks of the disease in domestic animals or humans.

"Pneumonic plague is the worst form," said Douglas. "It’s the one that you least want to get. You get sick fast and the chances of getting a rocky or even fatal course" are increased.

The plague is incredibly rare in the U.S. with an estimated eight infections in the country reported every year. Douglas said pneumonic plague is even rarer and accounts for just 3 to 5 percent of plague cases.

Douglas said the case shows the importance of considering all the options when diagnosing a patient, even extremely rare options like the plague.