Bubonic plague occurs primarily in areas where there are large communities of burrowing rodents.
This is because the rodents act as long term reservoirs of the disease. The virulence of Yersinia pestis is diminished in extremely cold and hot locations. Major areas of bubonic plague reservoirs are in China, eastern and southern Africa, the Western United States, and parts of Bolivia, Peru, Ecuador, and Brazil. Human infection usually occurs in areas of unsanitary living conditions where humans come into close contact with rats. Rural areas outside of cities, where conditions are poor and access to antibiotics is scare, usually are the focus of plague outbreaks.