Colorado health officials are investigating a suspected case in a human of the plague, the rare bacterial infection is infamously known for killing tens of millions in 14th century Europe. Today, it’s easily treated with antibiotics.

The Centers for Disease Control and Prevention estimates there are seven human cases of plague per year in the U.S., and in February, Oregon officials reported it in a person who likely got it from their sick cat.

Surprised to hear the plague is still around? Here’s what to know.

What is the plague?

The bubonic plague is the most common form of the bacterial infection, which spreads naturally among rodents like prairie dogs and rats.

There are two other forms of the plague: septicemic plague (which spreads through the whole body) and pneumonic plague (which infects the lungs).

Bubonic plague causes painfully swollen lymph nodes that are most commonly found in the groin, armpit and neck, called buboes. It will often advance and turn into the other two forms of plague if untreated.