There are two different strains of herpes simplex viruses. Herpes simplex virus type 1 (HSV-1) is usually associated with infections of the lips, mouth, and face. It is the most common herpes simplex virus among the general population and is usually acquired in childhood.

HSV-1 often causes lesions inside the mouth such as cold sores (fever blisters) and is transmitted by contact with infected saliva. By adulthood, up to 90% of individuals will have antibodies to HSV-1.

Herpes simplex virus 2 (HSV-2) is sexually transmitted and is usually associated with genital ulcers or sores. Individuals may, however, harbor HSV-2 and not have developed any symptoms. Up to 30% of U.S. adults have antibodies against HSV-2. Cross-infection of type 1 and 2 viruses may occur from oral-genital contact.

In addition to oral and genital lesions, the virus can also lead to complications such as meningoencephalitis (infection of the lining of the brain and the brain itself) or cause infection of the eye -- in particular the conjunctiva, and cornea.

A finger infection, called herpetic whitlow is another form of herpes infection -- usually affecting health care providers (because of exposure to oral secretions during procedures) and young children. The herpes virus may also infect the fetus and cause congenital abnormalities.

Herpes viruses can be transmitted to a newborn during vaginal delivery in mothers infected with herpes viruses (especially if the mother has active infection at the time of delivery, yet transmission may still occur at a lower rate without visible lesions).

The virus may be transmitted even in the absence of symptoms or visible lesions. Once the virus is acquired, it spreads to nerve cells and remains dormant. It may intermittently reactivate and cause symptoms (flares). Recurrences may be precipitated by overexposure to sunlight, fever, stress, acute illness, and medications or conditions that weaken the immune system (such as cancer, HIV/AIDS, and use of corticosteroids).