The plague never went away. Cases are reported in parts of Africa and the U.S. nearly every year

The plague, the rare bacterial infection infamously known for killing tens of millions in 14th-century Europe, never went away. It is now easily treated with antibiotics.

Since the 1990s, most human cases have been reported in Africa, especially Congo and Madagascar, according to the World Health Organization.

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There are an average of about seven human cases per year in the U.S., according to the Centers for Disease Control and Prevention, mostly in rural areas of the American West.

A worker at a plague research institute in Siberia recently died of pneumonia. The case has raised alarm in Russia after unconfirmed media reports suggested she might have died of pneumonic plague. Russian authorities say testing showed no evidence of plague, but the research institute has been quarantined.

"People think the plague is gone, it was something that occurred 500 years ago. It's not. It's here amongst us," said Simon Clarke of the University of Reading in the U.K. "Unless you get rid of those wild animals, which of course is not possible, then you're not going to get rid of the bacteria."

People in Siberia have been dying of plague for at least 5,500 years. The earliest evidence of a form of the disease was found in teeth from ancient hunter-gatherers north of the region's Lake Baikal.

Pneumonic plague and the more well-known bubonic plague — also called the Black Death — are caused by the same bacteria, called Yersinia pestis.

The bacteria spread naturally among rodents like prairie dogs and rats. Fleas that jump from infected animals and bite people can spread the disease.

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The bubonic plague is the most common form of the infection. It causes painfully swollen lymph nodes that are most commonly found in the groin, armpit and neck, called buboes. Other symptoms include sudden high fever and chills, headaches, and pain in the abdomen, legs and arms, according to the Cleveland Clinic.

There are two other forms of the plague: pneumonic plague, which can be caused by inhaling the bacteria, infects the lungs, while septicemic plague is an infection in the bloodstream. Both are fatal if not treated early, according to the World Health Organization.

Bubonic plague will often develop into the other two forms of plague if untreated.

Other than bites from infected fleas, people can also get plague by touching infected bodily fluids, so health experts recommend taking extra care when handling dead or sick animals.