The periodic outbreaks of syphilis in the United States have been blamed on societal changes like the sexual revolution and the crack cocaine epidemic. But researchers in London say something else is going on, too.

What the researchers wanted to know: What causes periodic syphilis outbreaks?

What they did: The researchers used data on both syphilis and gonorrhea, as reported to the Centers for Disease Control and Prevention; if syphilis outbreaks happen because of sexual behavior, they reasoned, gonorrhea should follow similar patterns. They focused on data from 1960 to 1993, looking at dozens of cities across the nation. They were looking for patterns—whether syphilis goes up and down periodically—and also at what might explain any such patterns.

What they found: Syphilis outbreaks are periodic, with an average of eight to 10 years between outbreaks; gonorrhea, for comparison, showed no such cycling. The researchers say this is most likely because of the biology of syphilis infection. As with many other diseases, after you've had syphilis, your immune system catches on, and it's harder to get it again. The immunity isn't perfect, but if syphilis is going around, eventually enough people will be immune that the disease slows its spread. Under this theory, another syphilis outbreak starts when there are enough new people who weren't around for the last outbreak. Infection with gonorrhea, on the other hand, doesn't give any protection against further infections.

What the study means to you: Social changes are still partly responsible for the outbreaks; for example, the researchers say syphilis and gonorrhea both declined in the 1990s because of the HIV epidemic (mortality increased and sexual behavior changed because of the virus). But the researchers say that those who point to recent increases in syphilis as evidence that gay men are becoming complacent about HIV are probably wrong; it's more likely that this is just the start of another cyclical outbreak, as more and more people arrive on the scene without immunity to syphilis.

Find out more: Learn the basics on syphilis from the National Institutes of Health.

Read the article: Grassly, N.C., Fraser, C., and G.P. Garnett. "Host Immunity and Synchronized Epidemics of Syphilis Across the United States." Nature. Jan. 27, 2005, Vol. 433, pp. 417–421.