Marlene Zuk, author of RIDDLED WITH LIFE, essay in New York Times

A Great Pox’s Greatest Feat: Staying Alive
By MARLENE ZUK
New York Times, April 29, 2008

The findings were hardly earth-shaking. They dealt with an obscure bacterial infection found in an equally obscure group of natives in Guyana. Nonetheless, they made headlines.
Why? Because the disease was syphilis. The new research suggested that syphilis originated as a skin ailment in South America, and then spread to Europe, where it became sexually transmitted and was later reintroduced to the New World.

The origin of syphilis has always held an implied accusation: if Europeans brought it to the New World, the disease is one more symbol of Western imperialism run amok, one more grudge to hold against colonialism. Sexually transmitted diseases have always taken on moralistic overtones — they seem like the price of pleasure. We tell ourselves that if we can just make everyone behave responsibly, we can halt the attack.

But we may not have as much say as we might like to think. Infectious diseases are caused by living beings that spread from one host to another, and natural selection will favor anything that increases that spread — say, a higher probability of becoming airborne, or a better means of attaching to the gut wall.

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.

Conventional wisdom used to hold that all diseases eventually evolved toward a more benign state, a “don’t bite the hand that feeds you” rationale. The muting of syphilis notwithstanding, we now realize that is not the case.

Diseases can evolve to become more virulent, more benign or neither — it all depends on what’s in it for them. For some diseases — cholera, for instance — killing the host is immaterial if the pathogen can spread via contaminated water sources. But sexually transmitted diseases must get around via sex. From the pathogen’s perspective, simply sitting around in the intestinal tract waiting for a too cursory bout of hand washing is unsatisfactory.

The disease organism from which syphilis arose is spread through simple skin contact. In chilly Europe, that’s too chancy a mode of transmission. Sex, on the other hand, is a fairly reliable means of transport, even for a delicate bacterium.

So you might blame Columbus, not for wreaking havoc on the New World through the spread of a sexually transmitted infection, but for wearing clothes. If he and his fellow Europeans had been more prone to going about au naturel, maybe the great pox wouldn’t have been so great after all.

Marlene Zuk is a biology professor at the University of California, Riverside, and author of “Riddled with Life: Friendly Worms, Ladybug Sex and the Parasites that Make Us Who We Are.”

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