Genital herpes can be caused by exposure to herpes simplex virus 1 (HSV-1), but most cases are caused by exposure to herpes simplex virus 2 (HSV-2). HSV-1 and HSV-2 can be found in and released from the sores that the viruses cause, but may also be released from skin that does not appear to be broken or have sores. The virus is secreted from oral or genital mucosa (moist tissue that lines some organs and body cavities) and may be transmitted through exchange of bodily fluids. Most skin is thick enough to serve as a barrier for the virus.

Genital herpes is most easily spread through having vaginal, oral or anal sex with another person who is infected with the HSV-2 virus and having an active outbreak. In addition, the disease is also easily transmitted during the few days prior to an outbreak. This period is known as prodrome.

It was once believed that transmission could occur only if the virus was active and causing symptoms, but this has since been proven false. There may be occasions when a person infected with herpes can transmit the virus without the presence of sores or other visible symptoms, known as asymptomatic reaction or asymptomatic shedding.

Genital herpes can also be transmitted when a person infected with HSV-1 has oral sex with another person. The virus can live outside the body for several hours, although there are few, if any, cases of genital herpes being spread through contact with contaminated objects, according to the National Institutes of Health.

Scientists do not know what causes the virus to become active. Some people report having an outbreak when they are sick, under stress or have been exposed to the sun. Some women also report having outbreaks during menstruation. In addition, poor diet, fatigue, friction, surgical trauma and steroid medications (e.g., asthma medications) may also trigger outbreaks. Some patients will never have a subsequent outbreak and others have frequent recurrent outbreaks.

Risk factors for genital herpes include:

• Multiple sexual partners. Having sex with more than one person results in a greater risk of acquiring genital herpes.
  
  
• High-risk partners. Having sex with a person who has multiple sexual partners or one who is infected with the HSV-2 virus puts a person at a higher risk of acquiring genital herpes.
  
  
• Unprotected sex. Having sex without using a condom. Condoms reduce, but do not eliminate, the risk of spreading the HSV-2 virus. Areas not covered by the condom can transmit the virus.
  
  
• Gender. More women acquire genital herpes than men. Women also tend to have more severe and longer-lasting symptoms.
  
  
• An impaired immune system. People infected with human immunodeficiency virus (HIV) or other diseases that affect the immune system are more likely to acquire genital herpes because of a decreased ability to fight off infection.