The highly contagious herpes simplex type 1 virus (HSV-1) is the usual cause of recurrent herpes labialis (RHL), known as cold sores by patients. Primary RHL usually appears on the lips and bordering areas. In rare cases, they may appear on the chin, face or nose.

HSV-1 can also affect the genital area through oral-genital contact. It is estimated that about 20% of genital herpes is caused by HSV-1.

A similar virus, herpes simplex type 2 (HSV-2), is the primarily cause of genital herpes. Although quite rare (estimated at <2%), HSV-2 can be associated with cold sores when there is direct oral-genital contact.

HSV-1 is made up of a core that contains its viral DNA (surrounded by a nucleocapsid) and an outer lipid-containing envelope. HSV binds to receptors on the host cell and the envelope fuses with the cell's plasma membrane. This fusion allows HSV-1 to release its viral DNA into the cell. Replication occurs through a complex chain of events that uses the host cell's machinery and cell nucleus.1,2

Viral replication results in cell lysis, which causes inflammation and the formation of blisters which fuse (coalesce) to form ulcers.

The virus will keep invading healthy cells in the area, leading to more and more infected cells and the development of a cold sore. The body's immune system responds and fights the virus back into remission. This can take anywhere from 7 to 10 days.

1. Whitley RJ, Roizman B. Herpes simplex virus infections. Lancet 2001; 357:1513-18
2. Corey L. Herpes simplex viruses. In: Braunwald E, Fauci AS, Kasper DL, et al, Eds. Harrison's Principles of Internal Medicine. 15th ed. New York, NY: McGraw-Hill Inc; 2001; 1100-6.

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