Over the past millennium the bacterium Mycobacterium leprae, which causes leprosy (Hansen's disease), has changed very slowly. Yet in less than a century it has given rise to strains resistant to a heavily used antibacterial treatment.

This genetic history has come to light with a little fishing—a technique known as DNA fishing, developed in part by geneticist Johannes Krause of the University of Tübingen in Germany. Beginning with old bones and teeth, the researchers trawl for ancient bacterial DNA using strands of contemporary DNA as “bait.” The old DNA that sticks to the bait is then studied through genetic sequencing. Krause and his co-authors detailed the work in June in Science.

Based on a pathogen's evolutionary history, researchers hope to spot the modern emergence of antibiotic-resistant strains. The data can also reveal when changes in human conditions—such as improved sanitation—influenced infection rates more than a pathogen's innate traits. These insights will be, according to University of South Carolina epidemiologist Sharon DeWitte, “important for understanding how diseases can evolve and what forms they might take.”

The next big target for DNA fishing, Krause notes, is M. tuberculosis, the bacterium behind the world's most widespread deadly infectious disease after HIV.

DNA fishing may miss ancient genetic fragments absent from modern strains, says Helen Donoghue, a microbiologist at University College London. But it could allow scientists to study even poorly preserved pathogen genomes from remote time periods. “As long as there are sufficient nucleic acids preserved in the specimen, there is really no limit,” says Alison Devault, a researcher at McMaster University studying ancient cholera. Soon the scourges of bygone centuries—and millennia—might be laid bare to help future generations avoid the worst torments of those past.