Scanning electron micrograph depicting a mass of <i>Yersinia pestis </i> bacteria (the cause of bubonic plague) in the foregut of the flea vector.

The plague might seem like a relic of the past, but the disease known as the “Black Death” or “Great Pestilence” that ravaged Europe in the Middle Ages, killing over 25 million people, is still a threat today.

Colorado officials have confirmed a case of plague in Pueblo County, following another case in Oregon.

Caused by the bacterium Yersinia pestis, which is typically spread by fleas and transmitted through small animals like rodents or cats, the plague has caused over 200 million deaths throughout history, dating back 3,800 years. While the majority of deaths occurred during three major pandemics – in the 6th century around Constantinople, in 14th century Europe, and in 19th century Asia – outbreaks have continued into modern times.

The reports that an average of seven plague cases are reported in the U.S. annually, primarily in the western and southwestern states. Globally, there are about cases each year, with the Democratic Republic of Congo, Madagascar, and Peru having the highest rates of the disease, according to the .

There are two main forms of plague infection: bubonic, which results from a flea bite or contact with the blood of an infected animal or material, and is characterized by swollen lymph nodes known as “buboes”; and pneumonic, a severe lung infection caused by , such as the coughs of infected humans or cats. Over 80% of plague cases in the U.S. have been the bubonic form, according to the CDC, but the pneumonic form is more dangerous.

Currently, there is no vaccine available in the U.S. to prevent plague infection. However, there are to reduce the risk of infection, including using insect repellent and applying flea control products to pets. Thanks to advances in treatment, including the use of readily available antibiotics, most plague cases today do not result in death. Nevertheless, untreated cases can be fatal. The overall risk of death from all types of plague in the U.S., according to , is around 11%.

Early medical attention is crucial for survival. Symptoms to watch out for include swollen lymph nodes, sudden fever, head and body aches, weakness, vomiting and nausea, shortness of breath, chest pain, and cough, especially if accompanied by bloody mucus.