There is evidence that the Mongol expeditions may have spread the bubonic plague across much of Eurasia, helping to spark the Black Death of the early fourteenth century. Italian historian Gabriele de’ Mussi wrote that the Mongols catapulted the corpses of those who contracted the plague into Caffa (now Feodossia, Crimea) during the siege of that city and how soldiers transported from there brought the plague to Mediterranean ports. However, this account of the origin of the Black Death in Europe remains controversial, though plausible, because of the complex epidemiology of the plague. Modern epidemiologists do not believe that the Black Death had a single source of spreading into Europe. Research into the past on this topic is further complicated by politics and the passage of time. It is difficult to distinguish between natural epidemics and biological warfare, both of which are common throughout human history. Biological weapons are economical because they turn an enemy casualty into a delivery system and as such were favored in armed conflicts of the past. Furthermore, more soldiers had died of disease than in combat until recently. In any case, by the 1340s, Europe faced a combination of overpopulation and famine. As a result, many had weakened immune systems, especially those living in squalid conditions. Whatever its origins, the Black Death killed around one third of the population in medieval Europe, or about 200 million people. The widening trade routes in the Late Middle Ages helped the plague spread rapidly. It took the European population more than two centuries to return its level before the pandemic. Consequently, it destabilized most of society, and likely undermined feudalism and the authority of the Church. In parts of England, for example, 80% of the population living in poverty were killed. Economic deprivation and war followed. In England and France, for example, a combination of the plague and the Hundred Years' War killed about half the population.