Herpes (HSV, for Herpes Simplex Virus) consists of two phases: the initial infection and recurrent outbreaks. Some never notice the initial infection, which can manifest with fever, headache, nausea, vomiting and very painful blistering lesions. Others have a very severe initial outbreak. The recurrent episodes happen at a variable frequency; some never have a second outbreak, and some have very frequent outbreaks.

It is generally considered that vertical transmission (passing along the virus to baby during the birth process) only occurs if there are active lesions at the time of delivery, although we know that sometimes the virus is passed along even if the mother has no symptoms at delivery. The most likely time to pass along the virus, however, is when the initial outbreak happens while pregnant. This is very uncommon. It is theoretically possible to infect the baby during recurrent outbreaks, but there have been no documented cases of this occurring.

If a woman has a history of HSV, it is imperative that she be checked for active lesions at the time of delivery, even if she has no symptoms. In the case of active lesions at the time of delivery, caesarian section is the best way to avoid passing along the virus.

If the baby does not become infected with the virus, an outbreak alone will not harm the baby.