That's why it's so common: As many as 50% to 80% of adults in the U.S. have oral herpes.

But few understand the relation between oral herpes and genital herpes, the sexually transmitted disease that affects more than 45 million adolescents and adults nationwide.

A blood test that shows antibodies to HSV-1 means you could have genital or oral herpes.

Genital herpes can be more difficult to diagnose than oral herpes, since most HSV-2-infected persons have no classical symptoms.

Type 1 (HSV-1) usually causes oral herpes, an infection of the lips and mouth.

(By comparison, experts estimate 50% to 80% of adults have oral herpes.)

Type 1 usually causes oral herpes, or cold sores.