Things are looking up for STDs these days. On the side of the newly-revitalized Christian right, you have abstinence doctrines strangling sex education and disease prevention efforts in schools (and celebrating the teen pregnancies that result). On the left, you have the “demystification” of non-lethal diseases like chlamydia, gonorrhea and HPV, sending the message that unprotected sex (and the infections that result) are “really no big deal.” Mix them together, and you’ve got a spike in U.S. infection rates, after years on the decline.

Granted, given that diseases like chlamydia and gonorrhea can be cured with antibiotics, and non-curables like herpes controlled with medication, it’s worth asking: Why are non-lethal STDs so dangerous?

Well, for one, because they can leave your body with permanent and serious damage.

And secondly, because they increase your risk of contracting far worse diseases, like HIV. We’ve known this for a while, and now a new study by Flemish researcher Teunis Geijtenbeek at the VU University Medical Center has figured out why. Using model replicas of human skin and vaginal cells, he found that gonorrhea, syphilis, and chlamydia, as well as yeast and bacterial vaginal infections, caused a change in Langerhans cells (LCs), which capture HIV. In disease-free skin, the LCs captured HIV but didn’t efficiently transmit the virus to T cells, thereby decreasing the chances of infection. But if the LCs were “activated by inflammatory stimuli” from an STD or other infection listed above, they became far better at capturing and passing on HIV.

Just how big was the HIV transmission increase? Geijtenbeek told DISCOVER that, while his results showed at least a tenfold rise, “the epidemiological data suggest that [the] chance of infection is increased ten- to one hundred-fold.” The risk jump lasts from the time of infection through the end of treatment.

And as for the argument that “HIV is no big deal these days“—if you seriously believe that, then there’s not much else to say. Just be sure to take those meds for the rest of your likely-shortened life—and have fun with those side effects.

Anti-transmission technologies are also seeing some success in the lab. At St. George’s University of London, a team of researchers led by Martin Cranage has been testing a rectal gel on macaques infected with SIV (the monkey version of the AIDS virus). They found that the gel, which contains the HIV drug tenofovir, partially or totally protected most of the uninfected monkeys from transmission.

Meanwhile, initial trials are already underway for an H.I.V. prevention pill, which would use one or more antiretroviral drugs—including tenofovir—to create a “pre-exposure prophylaxis” that would inhibit infection. The New York Times reports that as many as 15,000 people are expected to be participating in drug trials by mid-2009. Funding is coming straight from the U.S. Agency for International Development, the C.D.C., and N.I.H., as well as the Bill & Melinda Gates Foundation.

And now for the very bad news: Due to consistent underreporting, the infection rate in the U.S. is actually 40 percent higher than previously estimated. In other words, while the C.D.C. reported around 40,000 new infections in 2006, the actual number was more like 56,300, 53 percent of which were gay and bisexual men. The C.D.C. has known about its error since last October, but refused to release the new numbers until they were “published in a peer-reviewed medical journal.” Maybe they figured bad news sounds better coming from someplace else.