In parts of the Gobi desert, there exists plague foci (places where the disease lives naturally as a rodent infection). These places were isolated for most of history but from c1250, these places started to be passed through more regularly by Mongol caravans on new trade roots, marching soldiers and people riding the ‘Mongol express’. This made it possible for rats and their fleas to start travelling great distances through the dessert to more populated areas, and facilitated the spread of the plague, known, in its first European incarnation, as The Black Death.

Ibe al-Wardi, a contemporary Arab scholar from Syria, stated that the plague was in the East for fifteen years before it came to the West. In the 1330s, there are other references to the disease in the East and Chinese records talk of a mysterious illness that killed nine tenths of the population of Hopei, which some argue was the Black Death. The disease was still there in the 1340s, Gabriele de Mussis, a fourteenth century lawyer from Piacenza stated, “in 1346, in the countries of the East, countless numbers…. were struck down by a mysterious illness”.

From 1266, the Genoese held the port of Caffa (now known as Feodosiya), under a grant from the Mongols. In 1343, economic and religious tensions between the Mongols and the Italians erupted and they had a major confrontation. According to de Mussis, it all started over a street brawl that got out of hand when a Muslim was stabbed by an Italian. Kipchak Khan Janibeg, the Mongol leader, attacked the Italians and laid siege to them in the city of Caffa.

In 1346, the plague arrived on the western shore of the Caspian Sea, infecting nearby cities and towns. A year later, it travelled to the Crimea and attacked the Mongol army that were laying siege to Caffa. De Mussis wrote that, Janibeg, in what some believe to be the first instance of biological warfare,

ordered corpses to be placed in catapults and lobbed into the city in hopes that the intolerable stench would kill everyone inside…. Soon rotting corpses tainted the air… poisoning the water supply, and the stench was so overwhelming that hardly one man in several thousand was in a position to flee the remains of the Tartar arm. [In the medieval period, many believed that disease was spread by smell].

De Mussis may have invented the part about flinging corpses over the walls to cover the fact that God was not only attacking Mongols but also the Christians and infected rats may have carried the disease between the two armies.

However some historians believe the biological warfare scenario. Microbiologist, Professor Mark Wheelis argues that if flung, the bodies would be mangled and many of the defenders may have had cut or abraded hands due to dealing with the bombardment. In recent cases, twenty per cent of 284 plague victims caught it from direct contact. Wheelis also argues that when laying siege in medieval warfare, it was normal to camp a kilometre away to avoid enemy arrows and artillery, but the rat rarely strays more than 30-40 metres from its nest, which would have been in the Mongol camp for the food they could scavenge there.

In April or May 1347, the siege ended with many Genoese survivors fleeing westward and the Mongol army fading away. The plague almost certainly passed through other ports on the way to Europe but the Genoese of Caffa were generally seen by contemporaries as being responsible for bringing the epidemic to Europe. One thing that is not understood is how the Caffa Genoese and the bacteria they carried survived the 1600 mile journey to Sicily, which would have taken months to complete. The only explanation put forward by experts is that some of the people on the ships had ‘lucky genes’ and had some immunity to the disease and just acted as carriers.

The Italians then spread the diseases on their merchant ships and by autumn 1347, it had reached Byzantium, Rhodes, Cyprus and Messina. It probably first hit Britain on the 23 of June 1348 in Melcomb, on Weymouth bay, wiping out, as in the rest of Europe, an estimated one third of the population, a figure that translates to twenty-five million of seventy-five million European inhabitants.