Herpes simplex or herpes labialis is the infection of mouth, lips or gums. It is caused by the herpes simplex virus. The infection may lead to the development of small and painful blisters known as fever blisters or cold sores. Initially, a person does not experience any kind of symptoms or mouth ulcers. The infectious virus remains inactive in the nerve tissue of the face. Herpes simplex viruses are contagious. They can spread through personal or intimate contact with an infected person. It may also be caused through infected razors, dishes, towels and other shared articles. Also, oral to genital contact may lead to herpes to the genitals or vice versa.