Rats are off the hook when it comes to the Great Plague, say Christopher Duncan and Susan Scott.
Ask anyone about the Black Death or the Great Plague of London and they will say, while perhaps being a bit hazy about dates, that these were diseases caused by rats and fleas. This is in line with the popular theory that the plagues of Europe in the Middle Ages were the result of epidemics of bubonic plague - a disease that afflicts wild rodents in warm climates and is spread by infected fleas. Occasionally, the disease spreads to peridomestic rats, which are not resistant and die; the fleas then jump from their corpses and can infect passing humans.
But this interpretation of the plagues goes against common sense, a small amount of zoological knowledge and an inspection of the historical accounts of events during the 300 years of the epidemics.
When the Black Death appeared out of the blue in Sicily in 1347, people there quickly realised that it could be passed from person to person; they believed that only to look on a victim was sufficient to cause death. When the symptoms appeared, they were followed by about five days of the most appalling agony, frenzy and delirium. Michael of Piazza gives a valuable, graphic description: "The 'burn blisters' appeared, and boils developed in different parts of the body: on the sexual organs, in others on the thighs, or on the arms, and in others on the neck. The patient was seized by violent shivering fits, which soon rendered him so weak that he could no longer stand upright, but was forced to lie on his bed, consumed by a violent fever... The blood rose from the affected lungs to the throat, producing a putrefying and ultimately decomposing effect on the whole body." Soon Italy was overwhelmed and the disease then jumped across to Marseille and moved steadily northwards in an unstoppable wave of death. It reached the Arctic Circle just three years after it appeared in Sicily, leaving 40 per cent of the population of Europe dead in its wake.
The Italian health authorities were at the forefront of trying to control the disease and, even in the 14th century and with no medical knowledge, they accurately established a minimum quarantine period of 40 days. This became the gold standard for medical practice and proved to be completely reliable for the following 300 years. This shows that the pestilence (as the plagues were called) was passed through direct contact between humans because quarantine measures are ineffective against rat-borne diseases.
Why then is it popularly assumed that these devastating epidemics were outbreaks of bubonic plague? Bubonic plague had been around in Asia for centuries, but in the 1890s it erupted violently and caused grievous loss of life. Its biology was elucidated before the end of the century by pioneering work in Hong Kong and India.
Its diagnostic features are the swelling of the lymph glands (called buboes) and, because these were also found in some victims of the Black Death, everybody in about 1900 leapt to the conclusion that bubonic plague was also responsible for all the plagues of Europe in the Middle Ages. This view persisted for the whole of the 20th century. However, the real diagnostic feature of the Black Death was the haemorrhagic red spots on the chest that were called God's tokens - the result of bleeding from damaged blood vessels under the skin.
In our book Return of the Black Death , we have assembled overwhelming evidence that shows that bubonic plague was not responsible for the plagues of Europe. For example, during the 15th century, the disease came twice to Iceland, an island of snowfields and glaciers, and persisted through the winter. Rats did not arrive on the island until hundreds of years later, and it is impossible for fleas to breed in such severe weather conditions.
Once it is accepted that bubonic plague was not responsible for the epidemics that rampaged through Europe from 1347 to 1670, all sorts of things begin to fall into place. We can see how some plague tales have been embellished while many facts (such as the 40-day quarantine) have been ignored by researchers.
For the past ten years, we have studied scores of parish burial records that were kept during plague epidemics. The plague spread in an invariable pattern. It progressed readily within households but not between households. By studying the intervals between successive deaths in a line of infection, we were able to determine the duration of its incubation period; it proved to be a remarkably long 32 days and, for three weeks before the symptoms appeared, the victim was highly infectious. The time from the point of infection until death was therefore, on average, 38 days, which is in excellent agreement with the 40-day quarantine and with Daniel Defoe's perspicacious observation: "Because of its infectious nature, the disease may be spread by apparently healthy people who harbour the disease but have not yet exhibited the symptoms. Such a person was in fact a poisoner, a walking destroyer perhaps for a week or a fortnight before his death, who might have ruined those that he would have hazarded his life to save... breathing death upon them, even perhaps his tender kissing and embracings of his own children." What a pity this accurate description has been sidelined.
This incubation period was the secret weapon of haemorrhagic plague: even with the very limited transport available in the Middle Ages, apparently healthy people could spread the disease far and wide. During the age of plagues, it was quickly ascertained, particularly in France, that the danger came with the arrival of long-distance travellers. Trade was the engine of the plague. Haemorrhagic plague disappeared in 1670 as mysteriously as it had arrived three centuries before.
Where did the disease originate? Our research strongly suggests that haemorrhagic plague was a deadly virus that emerged from its animal host. The cradle for human evolution was around Ethiopia where Homo sapiens appeared between 100,000 and 200,000 years ago. Originally, the virus of haemorrhagic plague infected African animals, probably primates, but only in a mild way. Outwardly they were unaffected. Occasionally, it may have been transmitted to our hominid ancestors with lethal results, but the disease would not have spread far in these loose hunter-gatherer communities. Several Arabic sources mention the persistence of plague in Ethiopia and they tell how epidemics spread by caravan traffic from there to the Sudan and thence to Egypt and North Africa. Over history, haemorrhagic plague has repeatedly emerged from its animal host and attacked human civilisations with catastrophic results. A devastating epidemic originated in Ethiopia and struck Athens in 430BC; Thucydides wrote an eyewitness account of those terrible days, and the symptoms and behaviour of the victims correspond closely with the Black Death.
According to the historian Procopius, the plague of Justinian also originated near Ethiopia and moved down the Nile Valley in AD541 to the Levant and "seemed to spread all over the world; this catastrophe was so overwhelming that the human race appeared close to annihilation". In Constantinople, the death toll rose to a staggering 10,000 a day. Again, the symptoms correspond with those of the Black Death. The plague of Justinian continued for many years and, during the period AD541 to 700, it is estimated that the Mediterranean population halved. It led to the five great plagues chronicled in Islamic history, the first strike being in 6 and the last in 716, but Syria experienced further outbreaks every ten years from 688 to 744.
The plague then apparently lay largely dormant for 600 years before emerging from its base in the Levant, moving through Asia Minor and ravaging with a terrible loss of life the area round the Crimea in 1345.
Two years later, the world's greatest serial killer appeared in Sicily and began its 300-year reign of terror in Europe before suddenly disappearing.
But has haemorrhagic plague gone for good? More than 30 different lethal diseases have emerged since 1970 - many more than appeared in the preceding 3,000 years. And it is widely predicted by national health authorities that new diseases will continue to emerge with ever-increasing frequency because of our hectic lifestyles, jet travel and adventure holidays.
Is it possible that sometime in future that these factors will encourage haemorrhagic plague to emerge from its animal host again?
Christopher Duncan is emeritus professor of zoology at Liverpool University. Susan Scott is a social historian specialising in demography. Their book Return of the Black Death - The World's Greatest Serial Killer is published by Wiley.