Herpes is caused by two different viruses, which are active on a person’s skin. Not everyone with oral herpes or genital herpes knows when he or she is contagious. The herpes virus may be spread even when there are no visible lesions, through a process known as shedding.
As a result, a person with oral herpes, or cold sores as they are commonly called, who performs oral sex on a partner may transmit the virus to him or her. Likewise, a person with genital herpes may transmit the virus to a partner who performs oral sex on him or her.