Betting on protease-based regimens to stop the spread of HIV? No dice, according to a Harvard Med School report in last April’s AIDS Research and Human Retroviruses. Researchers revealed that low viral load in one’s blood does not equal low levels of HIV in semen. That’s because HIV invades semen-producing organs—testes, prostate and urethra—early on and creates its own subset of virus. “There’s a separate little trailer-park community of HIV in the testes that doesn’t react to drugs like the HIV in the rest of the body,” said the AIDS Treatment Data Network’s Richard Jefferys.

After eight men with HIV supplied semen and blood samples, it was found that viral levels in both differed dramatically (the man with the lowest level of virus in his blood had the highest level in his semen). Harvard researcher Ann Kiessling said one theory is that there might be a lag time between when HIV leaves the blood and when it leaves the semen. Regardless, she said, antiretrovirals must get on the ball. “Any therapy has to be designed to penetrate those organs.”

Since current treatments do not seem to impact the testes, Kiessling said, bear in mind that semen can be very infectious, despite a low viral load. As Jefferys said: “Don’t give up safer sex just because you’re undetectable.”