Genital herpes is caused by infection with either of two types of herpes simplex virus: HSV-1 or HSV-2. These viruses enter the body during sexual contact through small breaks in your skin or mucous membranes. HSV-1 usually causes oral herpes or “cold sores” but can be spread to the genitals of another person during oral sex while HSV-2 is the primary cause of most genital herpes infections. Most people who are infected with HSV continue to harbor the virus inside cells of the body for many years after the initial infection and may spread it to spread it others.

HSV-1 and HSV-2 virus particles are present in the sores that occur during an active outbreak of symptoms and can be spread by direct contact, but they can also be transmitted in the period between outbreaks when the skin does not have any visible sores. It is even possible for people who are infected with HSV but who never experienced symptoms to spread the disease to others.