Herpes simplex viruses include two distinct but closely related viruses, namely, HSV-1 and HSV-2. Both viruses can cause genital herpes. Roughly speaking, HSV-2 causes 90% and HSV-1 causes 10% of all genital herpes. Herpes simplex virus is a linear double-stranded DNA virus. HSV-1 and HSV-2 share approximately 50% homology of their genetic materials and they even express type-common surface antigens. It accounts for the high degree of cross-reactivity and the technical difficulty in differentiating the 2 viruses. On the other hand, HSV-1 & HSV-2 antibodies offer some degree of cross protection. Cell-mediated immune responses is more important than humoral responses in determining the severity of HSV infections. Hence, AIDS patients often have chronic and severe anogenital herpes. Genital herpes is usually transmitted by sexual intercourse. Direct inoculation of virus occurs through contact with infected secretions or mucosal surfaces. Orogenital contact with a partner with type 1 herpes labialis can also result in genital herpes. Asymptomatic shedding of HSV is the most common mode of transmission of genital herpes infection. It is estimated that more than half of the HSV-2 genital infections are asymptomatic.

Transmission is caused by close oral, anal, or genital contact, including intercourse, masturbation, kissing, or any direct skin-to-skin contact which allows for the transfer of bodily fluids.

A person is considered contagious when prodromal symptoms, active sores, and healing lesions are present.

Herpes is potentially contagious when no symptoms are present. That is, a person who has genital herpes is potentially always shedding active virus.

Approximately 1 in 6 members of the general infected population is thought to shed active virus occasionally without symptoms.

Some people do not get typical blister-like sores but harbor active virus in their saliva, vaginal, or penile secretions, and can shed the virus without knowing they have herpes.

Lesions can occur deep inside the vagina where they cannot be seen or felt, but can readily transmit the virus.

An uninfected individual has about a 75% chance of contracting herpes during intimate contact with someone actively shedding virus.

Oral herpes can be transmitted to the genitals, and vice versa. Symptoms are similar.

Auto-inoculation: An infected individual can spread the virus to other parts of his or her body by touching an area shedding virus and then touching, scratching, or rubbing another susceptible part of the body. Towels are especially conducive to this.

It is possible for a person to contract genital herpes if the partner with oral herpes performs oral sex. Oral herpes can be transmitted to the genitals, and vice versa. Symptoms are similar.

Environmental surfaces like toilet seats may be a source of contagion, but there is no evidence that this poses a real threat to the general population. Experts differ as to how long the virus can survive on its own. The primary cause of infection remains intimate contact.