Sero-Sorting, Ctd.

A reader makes some good points:

There may well be good evidence that serosorting is one of the factors keeping the rate of new HIV infections down, but the comparison with the syphilis rate isn't one of them - for two reasons.

Petrelis notes one of them in the blog entry you cite:

"What's not footnoted by DPH is the fact that in 1999 the CDC launched a national syphilis elimination effort, one that significantly drove up the number of syphilis tests performed and also enhanced surveillance."

If you look for syphilis and test for it more often and increase surveillance and contact tracing, then you're going to find more of it, regardless of what's happening with other STDs

The other was reported widely a year ago:

"A recent rise in syphilis rates in the United States is probably due to natural cycles rather than an increase in unsafe sex or other behaviors.... [Nicholas C.] Grassly and his colleagues argue in this week's issue of the journal Nature that syphilis infection follows a natural cycle that peaks at eight- to 11-year intervals. Though sexual behavior certainly influences the overall number of people infected, the researchers concluded, those regular ups and downs are an intrinsic property of the disease itself."

The article above goes on to note:

"Because [syphilis and gonorrhea] spread the same way, if changes in sexual behavior had caused the oscillating pattern in syphilis they should have created a similar pattern in gonorrhea. Yet gonorrhea rates show a steady rise from the 1950s through the 1970s, followed by a steady fall."

Because of the above, I'd be a lot more impressed by the slide both you and Petrelis highlight if it compared HIV infection rates with gonorrhea rates.