Kipchak Mongols besieging a Genoese trading centre on the Crimean coast catapulted their own dead into the city. The dead had been killed by a mysterious disease and the disease spread quickly in the besieged city. Some of the Genoese escaped by sea taking the disease with them. They landed at Messina in Sicily spreading the disease even further.
In June of 1348 the disease landed in England in Dorset and by the winter it had reached London. The deaths were at their peak in the summer of 1349 and it is estimated that somewhere between a quarter to half of England's population were killed. Even when the worst was over England was not safe and another outbreak of the disease occurred in 1361. More outbreaks of plague occurred until the final London plague of 1665.
The Black Death could have been what we know today as Bubonic Plague. Bubonic Plague is spread by the rat flea. The idea for a long time has been that the rat fleas spread across Asia and Europe on rats and people killing as they went. Initial symptoms of the bubonic plague appear between two to six days after being bitten by an infected flea. Flu-like symptoms, headaches, chills and a fever then begin. The plague bacteria congregate at a lymph node and multiple. The lymph node becomes infected and very painful. These symptoms are similar to those suffered by the medieval people who were infected by the black death.
The modern belief is that the Black Death was in fact a virus and not the bubonic plague. In 1918 a virus known as Spanish Flu spread across Europe killing somewhere between 20 and 40 million people, far more than were killed during the Black Death outbreak in medieval times. It seems easier to believe that a virus can be transmitted from person to person than a disease by flea bites especially when outbreaks of flu now occur regularly in the winter months.
Social class was not a barrier for the Black Death. Members of royal families and high ranking religious people also succumbed. These included: -
The population of Britain had been steadily increasing through the medieval period. After the Black Death had passed somewhere between a quarter and a half of the population was dead.