The U.S. Centers for Disease Control and Prevention (CDC) has revealed on Thursday that a dog with pneumonic plague has infected humans, a first in the history of the U.S.

Pneumonic plague is a very rare disease that is caused by the same bacterium responsible for the bubonic plague, which is known for causing the Black Death during the Middle Ages. The bacterium that causes pneumonic plague infects the respiratory system.

The dog, an American pit bull terrier, was infected with an illness marked by fever and jaw rigidity last summer. It was eventually euthanized at a vet's office in Colorado when its condition got worse. Four days later, however, the owner of the dog also went to the hospital with bloody cough and fever that worsened albeit the blood culture was initially misidentified.

Doctors first diagnosed the dog owner with pneumonia with a test showing that the Pseudomonas luteola was to blame. Some doctors, however, questioned the results because the bacterium responsible for the plague is often mistaken as P. luteola. A second test confirmed this suspicion when the man was found to have the pneumonic plague. He was given antibiotics and hospitalized for 23 days.

The dog's owner was eventually diagnosed of the disease. His friend and two other workers at the veterinarian's office also got sick. Blood samples taken from these four individuals as well as from the dog were found positive for Yersinia pestis bacterium that causes pneumonic plague. All were treated and have since recovered.

The CDC said that it was the first time that a dog to human transmission of the disease has been reported. Dogs are often asymptomatic or show mild symptoms of the disease and since the disease is primarily spread through sneezing, dogs are not likely to spread the infection to humans.

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

Although rare affecting only about eight people in the U.S. per year, plague is a potentially fatal disease. People often acquire the illness when they are bitten by infected fleas on rodents or if they had direct contact with the body fluids and wounds of infected animals.