Last summer, the United States had its worst single outbreak of pneumonic plague for 90 years. Only four people became sick and no humans died, but an assessment published by the Centers for Disease Control and Prevention (CDC) has sounded a warning based on the delay in identifying the disease. In doing so, they hope to prevent an ancient horror from returning.

Pneumonic plague is caused by the bacterium Yersinia pestis, the same species that causes bubonic plague. In the pneumonic form, it infects the lungs and is spread directly from human to human, rather than through fleas on rodents, making it capable of spreading even more rapidly.

The arrival of antibiotics has meant that either version of pneumonic plague can be controlled once identified. However, three of the four Colorado citizens infected last year were misdiagnosed, which could have had disastrous effects.

The outbreak started with a pit bull terrier who infected its owner, a veterinarian who treated it, and a staff member at the vet clinic. It is believed that the owner spread the disease to the fourth person, which the Tri-County Health Department, Colorado, and the CDC describes as, “The first instance of possible human-to-human transmission" in the United States for 90 years. However, it is possible the fourth individual also caught the disease directly from the dog, which has since been euthanized.

The CDC says that while dogs can host the fleas that spread the disease's bubonic version, direct transmission of pneumoic plague from canines has only been observed once before. “Y. pestis infection in dogs generally is either asymptomatic or the cause of only a mild, self-limiting febrile illness,” the report notes.

Identification was delayed because a sample of the first patient's bacteria was misidentified as Pseudomonas luteola. Fortunately, doctors involved with the case were aware that Y. pestis can be mistaken for P. luteola, and ordered a recheck. Nevertheless, seven days were lost before the error was corrected. “This delay resulted in the exposure of numerous medical personnel,” the report notes. The mistake was not an isolated incident. Three out of twelve cases of plague in the U.S. between 2010 and 2013 were initially misidentified in the same way.

Two of the four patients had relatively mild cases, possibly as a result of self-medication with antibiotics. While this was obviously good for them, the fact that they did not need to be hospitalized probably further contributed to delays in identification, and provided an extended opportunity for the disease to be transmitted further.

The CDC is urging doctors and vets to be on the lookout for pneumonic plague, particularly in the western United States where the disease is endemic among fleas carried by rats and prairie dogs. A handful of people are infected with plague each year in the U.S., but normally these happen as isolated incidents, with only one person infected at a time, and less than one case a year is of the pneumonic sort.

While almost half of those infected with bubonic plague survive, even without treatment, pneumonic plague is almost always fatal without treatment with antibiotics.