What are the causes for Oral Herpes (HSV-1) (Herpes of the Mouth)?

There are two types of herpes simplex viruses (HSV), they are termed HSV-1 and HSV-2. These two viruses have distinctly different DNA, and both cause oral and genital lesions. However, HSV-1 causes about 80% of all oral lesions and only about 20% of genital lesions while HSV-2 causes the reverse (about 80% genital and 20% oral). Studies also suggest that in adolescents, up to 40% of genital herpes is caused by HSV-1 because of reported increased oral/genital contact (transmission by oral sex).Oral herpes (HSV-1) infection (or exposure without noticeable infection) is common. About 65% of the U.S. population has detectable antibodies to HSV-1 by age 40. This article will focus on HSV-1, or oral herpes, not on HSV-2, also commonly known as genital herpes. HSV-2 is considered to be a sexually transmitted disease (STD). In addition, HSV-2 virus should not be confused with human papillomavirus (HPV), the cause of genital warts, and some cervical and other cancer types.
  • HSV-1 affects only humans. Mouth sores most commonly occur in children 1-2 years of age, but they can affect people at any age and any time of the year. Oral inflammation from HSV-1 is also termed herpes gingivostomatitis.
  • People contract HSV-1 by touching infected saliva, mucous membranes, or skin. Because the virus is highly contagious, a majority of the population is infected by at least one herpes subtype of HSV-1 before adulthood.
  • After HSV-1 infects a person, it has a rather unique ability to proceed through three stages.
    • Stage 1 -- Primary infection: The virus enters the skin or mucous membrane, usually through small cracks or breaks, and then reproduces. During this stage, oral sores, blisters, and other symptoms, such as fever, may develop.
      • The virus may not cause any sores and symptoms. People may not know that they have an infection. This is called asymptomatic infection.
      • Asymptomatic infection occurs twice as often as the disease with symptoms.
    • Stage 2 -- Latency: From the infected site, the virus moves to a mass of nerve tissue in the spine called the dorsal root ganglion. There the virus reproduces again, usually without any symptoms, and becomes inactive, until reactivated by certain body conditions (see stage 3).
    • Stage 3 -- Recurrence: When people encounter certain stresses (also termed triggers), emotional or physical, the virus may reactivate and cause new sores and symptoms. The following factors may contribute to recurrence: stress, ultraviolet light (including sunshine), fever, fatigue, hormonal changes (for example, menstruation), immune depression, and trauma to a site or a nerve region where previous HSV infection occurred.

    Source: http://www.emedicinehealth.com