NYT
April 29, 2008

Essay

A Great Pox’s Greatest Feat: Staying Alive
By MARLENE ZUK

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.”