Bubonic plague is a more common version of the disease and is rarely transmitted to humans.
pneumonic plague is the most virulent and deadly strain of the disease . It occurs in the lungs and any person who is infected can spread it to another person by sneezing or coughing near them. It can be cured with antibiotics, but it is always fatal if left untreated, according to the WHO.
In comparison, bubonic plague can be spread only from infected fleas or by treating the tissue of an infected animal.
Although the plague is inextricably linked to the 14th-century Black Death pandemic that has killed some 50 million people in Europe, it remains a relatively common disease.
The three most endemic countries are the Democratic Republic of the Congo, Madagascar and Peru.
An average of seven Americans get the plague. every year, according to the Centers for Disease Control and Prevention. In 2015, two people in Colorado died from the plague, and the year before there were eight reported cases in the state.
Jessie Jeng and Susan Scootie of CNN contributed to this report.