For about 199,950 of the past 200,000 years, our species had no idea that we were carrying HPV.
That’s not because HPV was a rare virus—far from it: a 2008 study on 1,797 men and women found
60 percent of them had antibodies to HPV, indicating they had been infected with the virus at some
point in their life. For the overwhelming majority of those people, the experience was harmless. Of
the estimated 30 million American women who carry HPV, only 13,000 a year develop cervical
cancer.
In this cancer-stricken minority, the peaceful balance between host and virus is thrown off. Each
time an infected cell divides, there’s a small chance it will mutate one of the genes that helps regulate
the cell cycle. In an uninfected cell, the mutation would not do much harm. But a cell that’s already
being pushed by HPV to grow faster is in a precarious state. What might otherwise be a harmless
mutation transforms an infected cell into a precancerous one. The cell multiplies much faster than
before. Its descendants grow so fast that the shedding of the top layer of epithelial cells is not enough
to get rid of them. They form a tumor, which pushes out and down into the surrounding tissue.
The best way to prevent most cancers is to reduce the odds that our cells will pick up dangerous
mutations: quitting smoking, avoiding cancer-promoting chemicals, and eating well. But cervical
cancer can be blocked another way: with a vaccine. In 2006, the first HPV vaccines were approved
for use in the United States and Europe. They all contain proteins from the outer shell of HPV, which
the immune system can learn to recognize. If people are later infected with HPV, their immune system
can mount a rapid attack and wipe it out.
The introduction of the vaccines has brought controversies of many flavors. The developers
recommend the vaccines for girls in their early teens. Some parents have protested that such a policy
promotes sex before marriage. In 2008, medical experts raised a different set of concerns in
editorials in the New England Journal of Medicine. It takes many years for HPV to give rise to
cancers, they pointed out, and so we don’t yet know how effective the vaccines will prove to be.
Another potential problem is the fact that current HPV vaccines only target two strains of the virus.
The choice makes a certain amount of sense for vaccine makers who have to balance costs and
benefits, since those two strains cause about 70 percent of all cases of cervical cancer. But we
humans are host to over a hundred different strains of HPV, which are constantly acquiring new
mutations and swapping genes between one another. If vaccines decimate the two most successful
strains, natural selection might well favor the evolution of other strains to take their place. Never
underestimate the evolutionary creativity of a virus that can transform rabbits into jackalopes or men
into trees.