Yersinia pestis, 1348. In the words of a contemporary chronicler, the year 1348 was the year of 'the great mortality', though there were other outbreaks in the 14c (1362, 1369, 1375). Some 25 million are thought to have died across Europe in the outbreak. It was spread by the fleas carried by rats. There were bubonic and pneumonic versions; a third, the septicaemic form, killed before the other two even had a chance to develop. The bubonic version showed as buboes, the swelling of lymph nodes; the pneumonic attacked the lungs; septicaemic poisoned the blood invisibly and fatally. Echoes of it survive in the children's rhyme 'Ring a ring of roses' and in the saying of 'Bless you' when someone sneezes. It reached England, probably Dorset, in June 1348; by January 1349 it had reached London. It is readily treatable today with antibiotics such as tetracycline, but not penicillin. In 1348 there was also a 'great *murrain' which killed large numbers of sheep. The price of everything fell: for example, a 40s horse sold for half a *mark, i.e. 6s 8d. By 1400, the population had approximately halved from around 5 million in 1300.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.