A rare form of disease with a deadly reputation has been diagnosed in a Colorado man, officials said on Wednesday, with it being the state's first confirmed human case of the illness in a decade.

An unidentified man from Adams County was discovered to have contracted pneumonic plague after his family dog unexpectedly died, with a necropsy concluding that the animal had been suffering from the same disease, the Colorado Department of Public Health and Environment said in a statement. "The plague is basically what's been here for centuries," said 9NEWS Medical Expert Dr. John Torres. "It's what caused the Black Death back in the Middle Ages."

Both the victims are believed to have been afflicted by the disease in an eastern Colorado county, but the department of health said the case was still under investigation, Reuters reported. Although the man is recovering in hospital, details on his condition were not released.

The bacteria that causes pneumonic plague occurs naturally in the western United States, particularly in Colorado, New Mexico, Arizona and California, according to the U.S. Centers for Disease Control and Prevention. It is transmitted by fleas to rodents, usually prairie dogs. "When the prairie dogs actually die, the fleas need somewhere to go. So once there is a prairie dog die-off, the fleas are out looking for somewhere to get a meal from," said Jennifer House of the Colorado Department of Public Health and Environment.

Pneumonic plague is the same bacteria that causes bubonic plague, but it infects a person's lungs, including symptoms of fever, headaches, shortness of breath, chest pains and a cough. Known to be the most serious form of disease, it is the only plague that can be transmitted person-to-person, usually through infectious droplets from coughing, Colorado health officials said.

Since 1957, 60 human cases of pneumonic plague have been identified in Colorado, with nine proving to be fatal, the health department said. "Although human cases occur infrequently, plague is severe and potentially life-threatening if not detected and quickly treated with common antibiotics," the department said.

Still, spraying insect repellant and staying away from wildlife, as well as making sure fleas that carry plague don't land on you are some ways to protect yourself from the life-threatening disease, experts stated.