The route was vulnerable to spreading plague. The Plague of Justinian originated in East Asia and had a major outbreak in Europe in 542 causing the deaths of a quarter of the Mediterranean's population. Trade between Europe, Africa and Asia along the route was at least partially responsible for spreading the plague. There is a popular theory that the Black Death was caused by the Mongol conquests. The claim is that the direct link that it opened between the East and West provided the path for rats and fleas that carried the disease. Although there is no concrete historical evidence to this theory, the plague is considered endemic on the steppe.