Why has San Francisco been so successful in bringing down HIV transmission rates? That's a good question for public health officials. Many now believe it is because of "sero-sorting." This is a technical term for sex between people of the same sero-status. So HIV-positive men stick to their own and vice-versa. In those contexts, using condoms is not as essential to curbing the epidemic. So check out this blog-post from Michael Petrelis, and one of the slides from a recent San Francisco Department of Public Health study:

Sero7

You'll notice an alarming increase in syphilis, but relatively stable rates of HIV transmission. The syphilis is not good news, obviously. But it suggests a silver lining: a lot of sex going on without condoms, yet without any spike in HIV rates. That's almost certainly because most of the people who get syphilis already have HIV. This isn't good for people with HIV (but syphilis can be treated effectively if detected). But the pattern does seem to be keeping HIV within the boundaries of the existing HIV-positive population. And that's good news for the uninfected.

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