(CNN) A pit bull was at the heart of a rare plague outbreak in Colorado that left four people sick last year. That's what a Centers for Disease Control and Prevention investigation found in a new report released April 30.

The CDC says on average about eight people get the plague every year in the United States. While it still can be life threatening, with modern medicine it is not the death sentence it was back in the Middle Ages when millions died. Antibiotics and antimicrobials can clear it up.

In last summer's incident, what stands out is the possible human-to-human transmission, according to the investigation. That hasn't happened in the United States since 1924. Earlier studies have shown that pneumonic plague, even its most severe form, can be transmitted person-to-person, but it is rare. And the dog-to-human transmission was unexpected according to the local health department. The team that investigated the case said that they could only find one other case of dog to human transmission in the medical literature. That was a 2009 case in China.

"There is no evidence this is a mutated strain or anything," said Dr. John Douglas, director of Colorado's Tri-County Health Department. "It is rare, and we don't know if it has simply been missed before or if it something about this particular breed of dog. We haven't been able to determine that."

Douglas did add they were not trying to malign a particular breed of dog. People with pets who live in parts of the country where plague is present in wild animals do need to make sure that they keep their pets far away.

In this instance, in at least three of the four cases, people got sick after being exposed to the infected dog. With the fourth patient it is "less certain" if her exposure came from the dog or the person.

The Colorado Department of Public Health and Environment first learned about the plague after lab samples showed the presence of Yersinia pestis the bacterium that causes the plague in blood taken from a middle-aged man hospitalized with pneumonia.

The man, who remains unnamed in the report, checked himself into the hospital after coughing up blood. The hospital initially diagnosed him with a different bacterial infection. An automated lab had misdiagnosed it. After six days in the hospital, the man's symptoms got worse, so they transferred him to another facility and ran further tests. It was only then that the lab figured out that he had the plague. He eventually recovered, but was hospitalized for 23 days.

When it was correctly identified, experts from the Tri-County Health Department looked into the case. What they found was that the patient's dog had recently been euthanized. The vet diagnosed the 2-year-old pit bull with hemoptysis, a condition where the dog would have been spitting up blood. The dog also had a fever and a rigid jaw. Only after additional tests did health care workers learn that the dog had been infected with the bacterium that causes the plague.

Investigating further, the department learned three other people who had contact with the dog and one person who had contact with both the dog and the initial patient were sick. A worker at the vet clinic had a fever and cough. An urgent care facility initially diagnosed her with bronchitis. When she didn't get better she started taking an antibiotic and took herself to the ER. Five days later she learned she was sick with plague. The hospital treated her with oral levofloxacin. Another worker at the vet's office got sick and was treated similarly.

The final patient was someone who had close contact with the initial man who got sick and she also came into contact with the body of the dog. She told investigators she had gotten the dog's blood on her hands. She was also exposed to the initial patient's blood after he had been coughing it up. She was hospitalized and was also treated successfully.

The health department learned a total of 114 people had close contact with either the dog or with one or more of the human patients. Some 88 of them were given antimicrobials as a precaution. Others were told to watch for fever. None of these additional 114 people got sick.

The CDC does investigative reports like this one for many disease outbreaks, the idea being that doctors and public health leaders can get a better sense of how to handle a similar public health problem in the future.

In this case, the report suggests that vets should consider plague when examining animals in areas where there have been plague cases with wildlife. Colorado has had cases in the past. Arizona, New Mexico and California have also seen modern plague cases . Often the cases come after a domestic animal has come into contact with prairie dogs who are infected. Typically, it spreads to humans after they've been bitten by infected fleas or if they've been exposed to bodily fluids that come from infected animals.

The report also suggests there are limitations to the automated systems hospitals use to test blood. In the initial patient's case, an automated system set to screen for different bacterium had missed the plague causing bacteria. "One of the take-aways from this is that the plague is still around and people need to be thoughtful about the diagnosis process," Douglas said. "We caught this because someone asked a thoughtful scientific question."