Leprosy in History

Leprosy has been familiar to humans for thousands of years, long enough for it to figure as a divine scourge or symbol of sin in several religious systems. But its prevalence has varied greatly across time. In particular, it seems to have been epidemic in Europe in the high Middle Ages, especially in the 12th and 13th centuries. Then it faded from prominence and by 1600 it was quite rare.

Bacteria are capable of evolving very quickly, and some forms that used to cause deadly disease are now much less virulent. So scientists naturally wondered if the changes in leprosy as a disease reflect genetic changes in the microbe that causes the disease, Mycobacterium leprae. A team led by Patrick Brennan of Colorado State extracted bacterial DNA from skeletons buried at medieval leper's cemeteries in England and Scandinavia, sequenced the DNA, and then compared it to DNA from modern M. leprae. The result: no difference.

So whatever caused the fluctuations in the incidence of leprosy in Europe, it was not changes in the organism itself. The authors suspect it was increasingly effective separation of lepers from the general population, preventing the spread of the disease.

Funny that a lot of the online links to this story have titles like "Scientists Unravel Mystery of Leprosy's Decline," whereas actually these scientists failed to learn anything about why leprosy declined, beyond ruling out evolution of the bacterium as the cause.