Robert J. Knell
Proceedings of the Royal Society of London B: Biological Sciences, Vol.271 (2004)
When syphilis first appeared in Europe in 1495, it was an acute and extremely unpleasant disease. After only a few years it was less severe than it once was, and it changed over the next 50 years into a milder, chronic disease. The severe early symptoms may have been the result of the disease being introduced into a new host population without any resistance mechanisms, but the change in virulence is most likely to have happened because of selection favouring milder strains of the pathogen. The symptoms of the virulent early disease were both debilitating and obvious to potential sexual partners of the infected, and strains that caused less obvious or painful symptoms would have enjoyed a higher transmission rate.
Introduction: Newly emerging infectious diseases are among the most important and conspicuous problems in public health. This is not a new phenomenon, and novel diseases have often caused tremendous mortality and misery in the past. In 1495, for example, a horrendous new disease appeared in Europe. At the time it went by a variety of names including the ‘Great Pox’ or the ‘French Disease’, but it is more familiar to us as syphilis.
Descriptions of the disease from this time make it clear that it was extremely unpleasant, and medical histories are filled with quotes from contemporary authors describing the horrors of the disease. A few years after its first appearance, however, the symptoms of syphilis were noticeably less severe, and within half a century it was widely observed that the disease was much less serious than it had initially been. Although this decline in virulence has been commented on before, it has not been examined with reference to modern ideas on disease evolution. Here, I briefly review the way in which the symptoms of the disease changed, and suggest possible evolutionary mechanisms that could explain the very high initial virulence of the disease and the subsequent decline in severity.