Herpes labialis is another way of referring to an oral herpes infection. Symptomatically, herpes labialis typically presents as an outbreak of blisters about the mouth and/or on the lips. Herpes labialis is most contagious during a symptom outbreak. Herpes labialis is usually caused by a herpes simplex virus 1 infection. Infection with herpes simplex 1 is exceptionally common: it's believed that roughly ninety percent of adults are infected with herpes simplex 1, though not all infections present symptoms. Herpes simplex 1 is likely the second most common viral infection, behind only infection with the virus that causes the common cold. Herpes labialis, or oral herpes, is not a significant threat to overall health.