The bubonic plague is the most common version of the disease and is rarely transmitted between humans.
Pulmonary plague is the most virulent and deadly burden of the disease. It has its origin in the lungs, and any infected person can transmit it to another person by sneezing or coughing. It can be cured with antibiotics but, according to WHO, it is always fatal if left untreated.
By comparison, bubonic plague can be transmitted only through infected fleas or through the handling of the tissue of an infected animal.
Although the plague is inextricably linked to the 14th century black-blooded pandemic, which killed some 50 million people in Europe, it remains a relatively common disease.
On average, seven Americans are affected by the plague each year, according to the Centers for disease control and prevention In 2015, two people died of the plague in Colorado, and in the previous year there were eight reported cases in the state.
Jessie Yeung and Susan Scutti of CNN contributed to this report.