Herpes is the general term for a group of viruses that cause blisters and sores. There are two types of herpes simplex viruses: herpes simplex virus 1 (HSV-1) and herpes simplex virus 2 (HSV-2).

HSV-1, the more common form of the virus, causes cold sores or fever blisters that appear on or around the mouth, lips or nose. Many people experience cold sores at some point in their lives. HSV-1 can cause genital herpes but it is less likely than the second form of the virus, HSV-2.

HSV-1 infection of the genitals can be caused by oral contact with the genitals or genital-to-genital contact with a person infected with HSV-1. Between 5 to 10 percent of genital herpes is caused by HSV-1, according to the National Institutes of Health (NIH). Genital herpes is more commonly caused by HSV-2, a sexually transmitted disease that causes sores in the genital area. It may be transmitted through vaginal, oral or anal sex.

Herpes viruses can live outside the body for up to several hours in a warm, damp place. It is possible for HSV-1 to be transmitted by an individual with a cold sore through kissing or by touching contaminated objects such as towels, bedding, utensils, toothbrushes or toilet seats. However, according to the NIH, there are few, if any, cases of genital herpes being spread through contact with contaminated objects.

HSV-1 and HSV-2 live in nerve cells. After the virus infects the body, it travels to the sacral ganglion (a sensory nerve root at the base of the spinal column). It can remain there in a dormant stage indefinitely. In some people, the virus reactivates and travels back to the skin, where it multiplies until it erupts at the surface in a sore. When this occurs, it is called an outbreak. The nerves in the genitals, upper thighs and buttocks are connected. As a result, a person can experience outbreaks in any of these areas.

Between outbreaks, the virus remains in the sacral ganglion, where the body’s immune system cannot destroy it. People with genital herpes develop antibodies (proteins used by the immune system to identify and neutralize foreign objects such as bacteria and viruses) in response to infection. But the antibodies cannot completely protect against different strains of the virus or reactivation of the dormant virus.

An estimated 90 percent of the U.S. population has been exposed to HSV-1, and 25 percent of the population ages 25 to 45 has been exposed to HSV-2, according to the NIH.

The U.S. Centers for Disease Control and Prevention (CDC) estimates that 45 million people ages 12 and older in the United States (one out of five adolescents and adults) have been infected with genital herpes. Between the late 1970s and the early 1990s, the number of Americans with genital herpes infection increased 30 percent. The infection rate remained stable over the 1990s.

In 1999, the estimated prevalence of genital herpes was 19 percent among the general population in the United States aged 14 to 49 years old.

Genital herpes is more common in women (about one out of four women is infected) than in men (almost one out of five). This is probably because transmission from men to women is more likely than women to men.

The herpes virus can be transmitted regardless of sexual orientation. For instance, it can be transmitted from woman to woman if a sore from one woman comes into contact with the oral mucosa (moist tissue that lines certain organs and body cavities) or the genital mucosa of her female partner.

Rarely, genital herpes can be passed during childbirth from a mother to a baby as it passes through the birth canal. This occurrence is uncommon because mothers pass antibodies to their babies during pregnancy. A woman who acquires genital herpes during the third trimester of pregnancy, however, is more likely to transmit the virus because she has not had time to build up antibodies.

Transmission of the virus to an unborn fetus can lead to premature birth. A baby born with herpes can also experience serious health problems, such as encephalitis (inflammation of the brain), severe rashes and eye problems. Herpes may also be life-threatening to an infant. According to the National Institute of Allergy and Infectious Diseases (NIAID), half of all babies infected with herpes either die or suffer from nerve damage.  

Although genital herpes can be uncomfortable, the disease is not considered a dangerous condition in adults. It does, however, increase the risk of acquiring and transmitting human immunodeficiency virus (HIV) by providing a point of entry or exit for HIV. In addition, HIV-positive people who are not being treated because their immune system is suppressed often have severe herpes outbreaks.

In addition to the two herpes simplex viruses, the family of herpes viruses includes varicella zoster, which causes chickenpox and shingles, and Epstein-Barr virus, which causes mononucleosis.