From here, however, the question of transmissibility gets more complicated. Acquisition of one type is more difficult-though certainly possible-if you already have the other type. This is because either type, contracted orally or genitally, causes the body to produce antibodies, some of which are active against both HSV-1 and 2. This acquired immune response gives some limited protection if the body encounters a second type. When a person with a prior HSV infection does contract the second type, the first episode tends to be less severe than when no prior antibodies are present.

On a practical level, this means oral HSV-1 is often the most easily acquired herpes infection. Usually the first herpes simplex virus that people encounter, oral HSV-1, is typically spread simply by the kind of social kiss that a relative gives a child. Because children have no prior infection with any HSV type, they have no immune defense against the virus.

By the time they're teenagers or young adults, about 50% of Americans have HSV-1 antibodies in their blood. By the time they are over age 50, some 80-90% of Americans have HSV-1 antibodies.

By comparison, almost all HSV-2 is encountered after childhood, when people become sexually active. Those who have a prior infection with HSV-1 have an acquired immune response that lowers - though certainly doesn't eliminate-the risk of acquiring HSV-2. According to one study (Mertz, Annals of Internal Medicine,1992), previous oral HSV-1 infection reduces the acquisition of subsequent HSV-2 infection by 40%.

A prior infection with oral HSV-1 lowers the risk of acquiring genital HSV-1 even further. Studies show that genital HSV-1 infections almost always occur in people who have no prior infection with HSV of either type (Corey, Annals of Internal Medicine, 1983).