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"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%."

Just a source.

http://www.herpes.com/hsv1-2.html

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