Genital herpes is caused by herpes simplex virus, which comes in two types. HSV-2 is the primary cause, responsible for an estimated 520 million genital infections worldwide. HSV-1, the virus most people associate with cold sores, also causes genital herpes and accounts for roughly 376 million genital infections globally. Together, more than 1 in 5 adults between 15 and 49 are living with a genital herpes infection.
Two Viruses, One Infection
HSV-2 spreads through sexual contact and is the classic cause of genital herpes. It’s responsible for the vast majority of recurrent genital outbreaks. Among people who experience symptomatic episodes, 92% have symptoms tied to HSV-2.
HSV-1 reaches the genitals mainly through oral-genital contact. Someone with a cold sore, or who is shedding the virus from their mouth without visible symptoms, can transmit HSV-1 to a partner’s genital area during oral sex. About 15% of genital herpes cases in the U.S. are caused by HSV-1, and that proportion has been increasing slightly each year. One practical difference: genital HSV-1 is far less likely to cause repeat outbreaks than genital HSV-2.
How the Virus Spreads
Transmission happens through direct skin-to-skin or skin-to-mucous-membrane contact. The virus enters the body through mucosal surfaces (like the lining of the genitals or anus) or through tiny breaks in the skin that you can’t see. It doesn’t require visible sores. HSV-2 is often transmitted when the infected person has no symptoms at all.
This is one of the most important things to understand about genital herpes: the virus sheds from the skin even between outbreaks. Among people with symptomatic HSV-2, the virus was detectable on about 20% of days sampled in research studies. Among people who had HSV-2 but had never noticed symptoms, shedding still occurred on roughly 10% of days, and 84% of that shedding was completely silent, with no sores or discomfort. This subclinical shedding is why many people contract or transmit the virus without realizing anyone was infectious.
What the Virus Does Inside Your Body
After entering through the skin, the virus replicates in skin cells at the site of infection. But it doesn’t stay there. It travels along nerve fibers, moving away from the skin and into clusters of nerve cells called sensory ganglia near the base of the spine. Once inside those neurons, the virus essentially switches off its active replication machinery and goes dormant. The viral DNA stays inside the nerve cell indefinitely, hidden from the immune system.
This is why herpes is a lifelong infection. The virus isn’t circulating in your blood where the immune system could clear it. It’s sitting quietly inside nerve cells in a state called latency, with most of its genes silenced by chemical tags on the surrounding proteins. Periodically, cellular changes inside the neuron reverse those silencing marks, allowing the virus to “wake up,” produce new copies of itself, and travel back down the nerve fibers to the skin surface. That’s a reactivation, and it can produce a visible outbreak or invisible shedding.
Triggers for Reactivation
Not every reactivation has an obvious cause, but several factors are known to provoke the virus back into activity. Psychological stress, fever, illness, sun exposure, menstruation, hormonal shifts, and physical trauma to the area (including surgical procedures) have all been linked to outbreaks. At the cellular level, stress hormones like epinephrine and corticosteroids can directly stimulate the virus to reactivate. Heat stress on nerve cells does the same in laboratory settings.
Another trigger involves nerve growth factor, a protein that helps maintain the health of the neurons where the virus hides. When nerve growth factor levels drop, the virus is more likely to reactivate. This may help explain why periods of physical stress or illness, which can disrupt normal nerve cell maintenance, tend to coincide with outbreaks.
Transmission During Childbirth
Genital herpes can also pass from mother to newborn during delivery. The risk varies dramatically depending on timing. A mother who contracts herpes for the first time near the time of delivery poses the greatest risk to the infant, with transmission rates as high as 60%. Her immune system hasn’t had time to build antibodies, and viral loads are high.
For mothers with recurrent infections, meaning they’ve had the virus for a while, the transmission risk drops to less than 2%. Their bodies produce antibodies that cross the placenta and offer the baby some passive protection, and the amount of virus shed during a recurrent episode is much lower.
How Condoms Affect Transmission Risk
Condoms reduce the risk of HSV-2 transmission, but the degree of protection differs by direction. In one study of couples where one partner had HSV-2 and the other didn’t, condom use reduced per-act transmission from men to women by 96%. Transmission from women to men was reduced by about 65%, though that estimate was less statistically precise. The difference likely reflects the larger area of exposed skin on the male genitals that a condom doesn’t cover, combined with the broader area of potential female shedding.
Condoms are most protective when used consistently, but because the virus can shed from skin not covered by a condom (the upper thighs, pubic area, or buttocks), they don’t eliminate risk entirely.
How Genital Herpes Is Identified
If you have visible blisters or sores, a healthcare provider can take a swab directly from the sore. This works best when the sore is fresh and hasn’t started crusting over. Swab-based testing is the most reliable method for confirming an active infection and identifying whether it’s HSV-1 or HSV-2.
Blood tests detect antibodies your immune system has made against the virus, which can confirm a past infection even when no sores are present. These have limitations. Antibodies take weeks to develop after a new infection, so testing too early can produce a false negative. False positives are also possible, particularly in people with a low likelihood of infection. Blood tests are not part of routine STI screening for most people, partly because of these accuracy concerns.