The reason seems to be, say researchers at the Hebrew University of Jerusalem and in London, that tuberculosis, a far more deadly disease, overtook leprosy, killing millions throughout Europe.
Their conclusion is based upon the examination of DNA from human remains from the ancient and medieval periods in Israel and Europe. In these examinations, the scientists found traces of both leprosy and tuberculosis bacteria in 42 percent of the cases.
The findings on the relationship between leprosy and tuberculosis were reported in a recent edition of the British Royal Society Proceedings B by Dr. Mark Spigelman, a visiting professor at the Hebrew University Faculty of Medicine and of the University College London; Prof. Charles Greenblatt of the Sanford F. Kuvin Center for the Study of Infectious and Tropical Diseases at the Hebrew University's Faculty of Medicine; and Dr. Helen Donoghue of University College London.
The earliest case of co-infection of both leprosy and tuberculosis was found by the researchers in the DNA from a body discovered in a 1st century CE burial cave in Jerusalem. This prompted the investigators to re-examine DNA samples from other ancient sites that they and their colleagues had worked on previously. In doing so, they found leprosy and tuberculosis bacteria in remains from a 4th century CE Egyptian shrine that was known to have been visited by lepers, from a 10th century burial ground in Hungary, and from a Viking-age cemetery in northern Sweden.
The conclusion drawn by the researchers from these multiple signs of co-infection of leprosy and TB bacteria is that those with leprosy, which was seldom fatal, were weakened to the extent that they became highly vulnerable to the "big killer," tuberculosis. This was exacerbated by the lives of deprivation that the lepers were forced to live as social outcasts.
Ultimately, the scientists theorize, so many of the lepers died of tuberculosis until there were too few of them to further spread leprosy. TB, meanwhile, was increasingly on the rise as people in the Middle Ages migrated to urban centers, where crowding and poor sanitary conditions provided a fertile breeding ground for the spread of the killer disease.
Today, tuberculosis, though curable, is still a major, long-term, epidemic disease, with millions of new cases reported around the world each year.