Plague is an infectious disease that is caused by the bacteria Yersinia pestis. Depending on lung infection, or sanitary conditions, plague can be spread in the air, by direct contact, or very rarely by contaminated undercooked food. The symptoms of plague depend on the concentrated areas of infection in each person: bubonic plague in lymph nodes, septicemic plague in blood vessels, pneumonic plague in lungs. It is treatable if detected early. Plague is still relatively common in some remote parts of the world.