About 10 per cent of Europeans enjoy a natural protection against Aids because of plague outbreaks that engulfed the continent in the Middle Ages, British scientists say. Researchers have known for some time that people with a genetic mutation known as CCR5-delta32 remain free of Aids even after contracting HIV. The mutation prevents the virus entering the cells of their immune system. The new theory suggests the CCR5 mutation is a byproduct of the plagues that ravaged Europe in the 14th century. The findings of Christopher Duncan and Susan Scott, researchers at the University of Liverpool in England, were published in the Journal of Medical Genetics. They claim that these plagues were not bubonic, but epidemics of viral haemorrhagic fever, which spared those carrying the CCR5 mutation, meaning future local populations contained a higher proportion of such people. Bubonic plague is a bacterial disease rather than a virus and cannot be blocked by a mutation. There appear to be higher levels of the CCR5 mutation the more recently a plague swept through a region. In Denmark, which endured an outbreak of plague as recently as 1711, the resistance level is even higher, at 15 per cent.