Plague, a disease that affects humans and other mammals, is caused by the bacterium, Yersinia pestis. Humans usually get plague after being bitten by a rodent flea that is carrying the plague bacterium or by handling an animal infected with plague. Plague is well-known for killing millions of people in Europe during the Middle Ages. Without prompt treatment, the disease can cause serious illness or death. The LcrV protein is a multifunctional virulence factor and protective antigen of the plague bacterium.