Four Colorado residents were infected with the plague and they got it from a dog, according to the Centers for Disease Control and Prevention (CDC).

In 2014, a 2-year-old American pit bull terrier fell ill with a fever and symptoms like rigidity of the jaw and drooling. The sickness progressed so rapidly that the dog was euthanized the next day at a local veterinarian's office.

The dog's owner presented at the hospital four days later with a fever and a bloody cough. The first blood culture test was misidentified and had to be redone, according to the CDC. When the results of the second test came back, it resulted positive for Y. pestis, also known as pneumonic plague. When the dog's remains were tested, Y. pestis was found.

"Frankly one of the biggest surprises of this outbreak is the source," said study author John Douglas, of Tri-County Health Department in Colorado, according to ABC News. "Primarily ... dogs don't get sick at all or they get a minor illness," after being infected.

Lead study author Janine Runfola, also of the Tri-County Health Department, told ABC News that cats are more likely than dogs to infect humans. Cats tend to develop more plague symptoms than canine companions. "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 told ABC News. "The cat gets sick and sneezes and coughs on its owner."

The dog's owner spent 23 days in the hospital. A friend of the owner and two veterinary employees were treated after exhibiting symptoms. All four individuals have fully recovered, according to the CDC.

"Pneumonic plague is the worst form," Douglas told ABC News. "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 greater.

The plague is rare in the United States with only about eight cases reported each year and pneumonic plague counts for about 2 percent of those cases, according to the report. Pneumonic plague has a high fatality rate of more than 93 percent. "Early recognition of plague, especially the pneumonic form, is critical to effective clinical management and a timely public health response," study authors wrote.

Douglas told ABC News that this case highlights the importance of considering every option when diagnosing a patient. Sometimes the least likely bacterium is the culprit.