The Plague disease, or Black Death, caused by Yersinia pestis, is enzootic (commonly present) in populations of ground rodents in Central Asia. reported the origin of the plague bacillus to be in China. An older theory places the first cases in the steppes of Central Asia, and others, such as the historian Michael W. Dols, argue that the historical evidence concerning epidemics in the Mediterranean and specifically the Plague of Justinian point to a probability that the Black Death originated in Africa and spread to Central Asia, where it then became entrenched among the rodent population.