DENVER (Reuters) - Three more people in Colorado have been diagnosed with the plague after coming in contact with an infected dog whose owner contracted a life-threatening form of the disease, state health officials said on Friday.

In all, four people were infected with the disease from the same source, the Colorado Department of Public Health and Environment said in a statement.

Last week the department said a man in an eastern Colorado county whose dog died of the plague had been diagnosed with pneumonic plague, a rare and serious form of the disease.

The man remains hospitalized, but authorities have not released his condition.

The three people in the latest reported cases had “mild symptoms” and have fully recovered after being treated with antibiotics, the department said, adding that they are no longer contagious.

Two of the patients in the new cases contracted pneumonic plague, the department said.

Pneumonic plague is the only form of the disease that can be transmitted person-to-person, usually through infectious droplets from coughing.

The bacteria that causes plague occurs naturally in the western United States, primarily in California, New Mexico, Arizona and Colorado, according to the U.S. Centers for Disease Control and Prevention.

The infected canine in Colorado likely contracted the disease from prairie dogs or rabbits, which are the primary hosts for fleas that carry the bacteria.

When an infected animal dies, the fleas spread the disease when they find another host.

Colorado has seen a total of 12 cases of humans infected with the plague over the last decade, said Jennifer House, the department’s public health veterinarian.

“We usually don’t see an outbreak like this related to the same source,” House said.

Colorado had not had a confirmed human case of pneumonic plague since 2004, she said.