HSV-1 is closely associated with orolabial infections and HSV encephalitis. HSV-1 serum testing is particularly useful for the follow-up of pregnant women, who were not previously exposed to HSV-1 and consequently are not protected against the virus. The presence of HSV-1 IgG antibody in serum is an indication of previous exposure. A significant increase in HSV IgG is an indication of reactivation, current or recent infection. The presence of HSV-1 IgM antibody in serum is an indicator of active infection.