Say you have genital HSV-1 and your partner has genital HSV-2. If you have unprotected sex, there is a small but real risk that you will get HSV-2, resulting in more outbreaks and more shedding. Doctors have documented cases where a person acquires HSV-2 after a prior genital HSV- 1 infection, they don't think it happens often, but it does happen.

On the other hand, it's very unlikely that your partner will get genital HSV- 1 from you. A case of a person acquiring HSV-1 on top of HSV-2 is possible, but it would be unusual.

What if your partner has genital HSV-2 and you perform oral sex on him or her? Will you get HSV- 2 in the mouth? Given the widespread practice of oral sex in the lifestyle and the prevalence of genital HSV-2 infection, you might expect oral HSV-2 to be relatively common. It's not.

According to one study, almost 100% of recognizable HSV-2 infection is genital (Nahmias, Scandinavian Journal of Infectious Diseases Supplement, 1990). One reason is that most adults are already infected with HSV-1 orally, which provides some immunity against infection with HSV 2. Another reason is that oral HSV-2 rarely reactivates, so even if an infection does exist, no one knows.

So far we've been talking about transmission of HSV-1 (oral) or 2 (genital) from its site of preference. What about transmission from another site? Say you acquire genital HSV-1 through oral sex. Can you spread the virus to a partner through genital sex?

The answer is yes, but probably not as easily as it was spread through oral sex. The main reason is that the virus reactivates and sheds less often outside its site of preference. Only about one quarter of people with genital HSV- 1 shed virus at all in the absence of symptoms, while 55% of people with HSV-2 do (Wald, New England Journal of Medicine, 1995). "Shedding data appear to parallel recurrence data, meaning that people who have a lot of recurrences also have a lot of shedding," says Wald.

While HSV- 1 can be spread from genitals to genitals, "we think it is spread more easily through oral sex because HSV-1 reactivates more frequently in the oral area," says Wald. However, she warns, "transmission of genital HSV-1 during asymptomatic shedding has been documented." In other words, genital HSV-1 can be spread through genital sex, even when there are no symptoms.

If HSV infection is as easily transmitted from the mouth as from the genitals, then why do people take steps to prevent genital but not oral infection? Why don't we kiss through dental dams ?

"It's ironic, isn't it?" says Wald. "It's not about health, it's about social acceptability."