Bubonic plague
A close look at the bacteria Yersinia pestis which causes the plague. Wikipedia

A sick Colorado pit bull infected its master, two veterinarian workers and possibly also a friend of the owner with plague, in the first-ever transmission of its kind, health authorities have revealed.

The dog was euthanized over a year ago after becoming weak and coughing up blood. The owner was the first to come down with plague, and became seriously ill, according to a report released by the federal Centers for Disease Control.

The two vets soon came down with a respiratory infection that they quickly treated with antibiotics, eradicating what turned out to be a mild case of the plague. The three illnesses, the pneumonic form of the disease that infects the lungs, were linked to the dog in a CDC investigation. A female friend of the dog owner is also believed to have contracted the disease, either from the dog or the owner because they were both coughing up blood. All four made a full recovery.

The case is unique because it's the first time a dog has been identified as a source of human plague in the US. It's also the first possible human-to-human transmission since an outbreak in Los Angeles in 1924.

The Yersinia pestis bacteria, that causes Bubonic plague, is passed along in flea bites. The pneumonic form, that infects the lungs, can either be a complication of Bubonic plague or transmitted directly by cough droplets. It's the most dangerous form of plague because it can spread in the air, and it's the only way plague is transmitted from human to human.

"Pneumonic plague is the worst form," Dr. John Douglas, director of Colorado's Tri-County Health Department, 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 increased.

The unusual case serves as a warning to health officials and vets to be on the lookout for plague symptoms. The disease is treatable with antibiotics if diagnosed in time.

Plague, which wiped out tens of millions of people in the middle ages, continues to be a concern in the western US, where the bacteria infects wild rodent populations. An average of eight people a year contract the disease from fleas.

"Plague is virtually always confined in this day and age to rural regions in the West," Douglas told NBC News. "That is because the vector of plague is typically the prairie dog, although there are other rodents that can transmit as well."

"Don't let your dog run around where the prairie dogs are," Douglas warned American pet owners. "If you live in the West and you live in places where there are rodents or you are hiking ... you need to be aware. Pets should get flea treatments and be kept away from wild animals."