Black Death was an outbreak of infectious disease that swept the whole of Eurasia, killing a large part of the population.

It is better known than a similar occurrence in 541-2 in the time of Justinian.

Earlier we have the account of a plague in Athens in 432 BCE recounted by Thucidydes. That plague affected the outcome of the Peloponesian War. Thucidydes believed it had started in Africa. Modern epidemiologists believe it might have been Typhus, but there is uncertainty.

Epidemiologists try to study these episodes in human history.

Historical problems
How much do we really know about this event?

1. Where did it start?
The movement of the disease can be traced exactly, as the chronicles give dates of arrival in different places.
Central Asia 1331-2
The first report was in the nomadic steppes. It is not certain whether it arose there but it probably spread from there to India, China and the west. It entered Europe from Central Asia via a battle in the Crimea between Genoese merchants and "Tatars" in 1345-6.
The Genoese brought the disease to Constantinople in 1347 and it reached Italy the same year carried by trading ships.
It crossed the Alps in 1348, arriving in France and England in the same year.
The disease may have entered England first via Melcombe (Weymouth), Bristol or Southampton, but quite possibly through all three ports in ships coming from the English-owned wine area of Bordeaux (Gascony).

2. What was the disease?
Because bubonic plague (caused by the bacterium Yersinia pestis) is known to have killed many people in later epidemics, up to the 19th century, it has been assumed that the 14th century disease was the same. But the descriptions in the chronicles are obscure and may be the uninformed observations of a different disease. According to chroniclers printed in Horrox the Black Death killed animals and birds as well as humans, whereas Plague was more specific and did not kill cats and dogs - though it did kill Black Rats, whose bodies were not reported, unlike later Plague epidemics.

Some modern writers have suggested a disease like Ebola. The death of birds suggests Bird Flu. The fact that Iceland was affected, even though there were no rats there at that time, also suggests Yersinia was not the agent.

Here is a discussion of the virus theory.

This has implications for our modern society. Although the Plague bacillus is present in wild life, even in parts of the United States, it is very unlikely to cause an epidemic in modern times because it can be treated with antibiotics (at least until recently), and people live much more hygienic lives - not in contact with rats and their fleas. However, if Black Death was a virus like Ebola or Marburg, a repeat is possible. At present these viruses are so rare that there is no treatment or vaccine. A really infectious and lethal agent could have the same effect on modern society as Black Death did, but kill many more people. If it spread via air travel it could infect every continent almost simultaneously. If it killed even the lowest of estimates for the death rate (see below) it could kill thousands of millions of people.

The World Health Organisation and the Center for Disease Control in the United States discuss such a possibility and prepare even for milder epidemics such as the "influenza" at the end of the first world war which killed more people than in the war itself (shown to be genetically very similar to the Bird Flu currently - 2007 - found in commercial bird flocks in several parts of Eurasia and Africa). SARS, a disease that originated in China, rapidly spread to Europe and Canada and killed several people before being isolated.

Outbreaks of Ebola in Africa are tackled by teams of WHO professionals equipped with modern technology in the form of isolation suits - but still many people die. Ebola itself may not be a danger as it kills people so quickly that it doesn't spread easily. Black Death can be shown from records to have had a longer incubation period allowing it to spread at walking pace, as people carrying the disease agent could travel to the next town in the refugee columns. English law recognised that 40 days was a necessary quarantine period (as the word tells us - French for 40).

2011 Archaeologists have retrieved examples of bones from the Black Death period and have extracted the DNA of Bubonic Plague bacteria. Scientists will attempt to discover whether the variety of Yersinia pestis they are examining had any qualities that made it more virulent than modern varieties. See this article:New Scientist

3. How many people died?
The people at the time wrote possibly exaggerated estimates of the number of deaths, which are probably not accurate. For example a survival rate of one in ten as stated by some chroniclers, is hard to believe - though some areas do seem to have had a higher death rate than others, and there were areas almost immune. Modern researchers are trying to find a believable estimate.

Estimates at the time were in the range of one in ten survivors, to one third. Modern estimates are that there was about 55% mortality, reducing the population to about a half. Two million people may have died in England. A modern epidemic of this virulence would kill about 3000 million people.

An interesting line of modern research has suggested that about 10% of European populations possess a gene that may have been selected for by Black Death, giving immunity to HIV. That suggests that the survivors were genetically predisposed to resistance. African populations are believed to have the gene too but almost nothing is known about whether the disease spread south of the Sahara.

Rosemary Horrox - Black Death
original documents - essential reading

The Black Death: A Personal History

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Was it Haemorrhagic fever, spread by virus, like Ebola?
Sue Scott et al. - The Biology of Plagues


Biology of Plagues: Evidence from Historical Populations

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Bubonic plague

4. What were the after effects?
In every village and town there were empty houses, land that could not be worked, a shortage of people to fill positions, including priests, bishops, abbots, and lawyers. Some villages were abandoned and their sites are known to this day. One example is Knowlton in Dorset whose ruined church remains today. In Wimborne Dorset the old town was abandoned - its site remains a field to the present day - and a new town built on a nearby site.

Wages rose
The lords of the manor wanted the labourers in the villages to continue to work as serfs for nothing on the lord's land as part of their manorial obligations. Increasingly the surviving peasants felt able to defy the lords by moving away, either to the towns or to another lord. There is a record of serfs simply refusing to work for nothing. In effect the wages of the labourers rose by ordinary market forces as landowners were prepared to pay them more to get in the harvest. The upper classes deplored this affront to what they regarded as the natural way of things, but they could not prevent it, even though laws were passed forbidding relaxation of the conditions of serfdom (Statute of Labourers 1351). Serfdom was in practice fatally weakened throughout western Europe - and especially in England (eastern Europe was a different story, but there the death rate seems to have been much less because of low density of population).

Prosperity increased, as can be seen in the large churches built in the wool districts of East Anglia.

Weakened Church
In western Europe there was a huge turnover in the priesthood with an influx of new, illiterate, priests. The prestige of the church fell because of its powerlessness to influence the course of the epidemic. This may have sown the seeds of discontent that led to the Protestant Reformation in the following century. Henry the eighth's enquiry into the condition of the church showed many illiterate priests (mumpsimus), perhaps showing that the Church never recovered from the damage, even 200 years later.

Language
The English language itself changed with the Great Vowel Shift occurring, possibly because of the influx of people from the countryside to replace the people who died in the towns. That's why spelling, developed before the Black Death, is often different from present day pronunciation.

Causes
One view, difficult to investigate, is that population pressure before Black Death had depleted the soil of minerals and humus, and that what the soil needed was a fallow period. Another, more certain, factor is that bad weather associated with the beginning of the Little Ice Age - a series of bad harvests from rainy summers from 1315 onwards - had caused poor health from lack of food after a series of poor harvests so that people's immune systems were weakened. There was in fact a famine preceding the epidemic, with outbreaks of other diseases. The population may therefore have been medically weaker because of poorer food and so vulnerable to infection. The drop in population provided a rest period for the land for over a century, and so the health of the survivors was consequently better. Forests came back on abandoned land. It is difficult to know how these assertions could be tested, though examination of graveyards is constantly producing new knowledge. Examining the bodies in Plague cemeteries shows the effects of the 1315 famine in the victims. It is often said that the English diet in the 15th century was the best in known history - that was the time of the "Roast Beef of Old England".

Could there be a repeat?
The emergence of antibiotic-resistant varieties of Yersinia pestis means that a breakdown of hygiene might produce an epidemic that could not be easily controlled if there were other breakdowns in society, such as a lack of medical services. If the original was Marburg or Ebola there are still possibilities of a worldwide epidemic.

What this suggests is that while modern society is functioning normally an epidemic is unlikely, but an unexpected event such as a major nuclear war or a hypervolcanic explosion could produce the conditions for an outbreak of infectious disease.