The syphilis bacterium, Treponema pallidum, has no nervous system or brain, no consciousness with which to plot an attack. But it has an ability that is even better: it can reproduce at a rate that leaves us in the evolutionary dust. For any S.T.D., making the host more likely to have sex will benefit the pathogen that causes it. And syphilis may be a case in point.

Detailed records of syphilis infection start appearing in Europe from 1495, and a fearsome disease it was. Smallpox was called smallpox to distinguish it from the great pox, syphilis, which evoked this description from Ulrich von Hutten in 1519: “Boils that stood out like Acorns, from whence issued such filthy stinking Matter, that whosoever came within the Scent, believed himself infected. The Colour of these was of a dark Green and the very Aspect as shocking as the pain itself, which yet was as if the Sick had laid upon a fire.”

Two points are noteworthy about this vivid account. First, it contrasts markedly with modern experiences with the disease. Although serious in its overall effects  which can include heart problems, brain damage and infertility  the rash and other overt symptoms of syphilis are now much more muted, and the disease may go undetected for some time, which helps explain why it is so hard to control. Second, it is reasonable to suppose that a sufferer of the symptoms von Hutten describes would be unlikely to get a lot of dates.

These two observations led Rob Knell, a scientist at Queen Mary University in London, to propose (in a 2004 paper in The Proceedings of the Royal Society of London) that they were connected. If a syphilis-ridden individual were less likely to have sex, and hence spread the disease, it would behoove the disease organism to evolve a less acute effect on its hosts. Syphilis became less severe, he argued, because it was transmitted more readily if victims were still attractive to the opposite sex.

And while these changes were too rapid to be attributed to humans’ evolving resistance to the disease, he continued, for the syphilis bacteria, even a few years represents many thousands of generations. So we have syphilis itself to thank for the lessening of its symptoms. The disease is still serious, of course. But the rapid evolutionary change is striking.