Scientific research now suggests, however, that the terrible suffering of our forebears means that a significant proportion of modern Europeans is resistant to Aids.

Research by two British biologists published in the Journal of Medical Genetics suggests that around 10 per cent of Europeans enjoy such protection as a direct result of the series of plagues that swept across the Continent from the Middle Ages onwards.

Biologists have known for some time that people carrying a particular genetic mutation, known as CCR5-delta32, remain free of the disease. The mutation prevents the HIV virus from entering the cells of the immune system.

It is also a continuing puzzle as to why the strains of HIV that have swept through Africa have made much less of an impression in Europe.

The new theory suggests that the CCR5 mutation was a by-product of the European plagues. The proportion of people carrying the natural resistance rises dramatically in Europe and particularly Scandinavia, where the figure is 14-15 per cent. It is relatively low in countries bordering the Mediterranean and not found at all in sub-Saharan Africa, Asia or among native Americans.

Christopher Duncan and Susan Scott, based at the University of Liverpool’s School of Biological Sciences, suggest that the natural protection from HIV can be attributed to the peculiar history of the plague from classical antiquity to 18th-century Denmark.

They have put forward a mathematical matrix showing how the frequency of genetic mutation rises with each stage of plague outbreak from the Black Death in 1347 to the Great Plague of London (1665-66) and beyond to the Plague of Copenhagen more than half a century later.

Using a computer model, they demonstrate how the pressure of natural selection increases the number of those carrying the resistance from around one in 20,000 at the time when plague devastated Europe, particularly France, during the mid-14th century to one in ten three centuries later.

Professor Duncan and Dr Scott, authors of The Return of the Black Death, published last year, are adamant that these plagues were not bubonic but epidemics of viral haemorrhagic fever that used the CCR5 receptor as the “entry port” into the immune system. These lethal haemorrhagic fevers — whose modern version is Ebola fever — are believed to have occurred as far back as antiquity.They were recorded in the Nile Valley from 1500 BC and in Mesopotamia (700-450BC), Athens (430BC), the plague of Justinian (AD 541-700) and the plagues of the early Islamic empire (AD 627-744).

If the plague struck a village carrying off half the population, those with the natural resistance became a higher proportion of the survivors. Then they handed it on to their descendants, who were also more likely to survive a plague.

Professor Duncan dismisses other theories which claim that resistance to Aids can be attributed to smallpox or bubonic plague. He is less sure that his work will have any practical impact on medical research and is not hopeful that the genetic mutation, can be reproduced artificially to create protection.

GENETIC RISK OF DISEASE

Particular genetic traits, arising either by natural selection or by accident, are known to influence risk of getting diseases

• Malaria: in areas where malaria is endemic, more people are born with genes that confer protection
  
  
• vCJD: all cases of the human version of “mad cow” disease occurred in people with a particular genetic make-up, shared by about 40 per cent of population
  
  
• Alzheimer’s and Parkinson’s disease: inheriting a version of a gene APO-E can increase the risk of Alzheimer’s

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