When herpes flares up again, it is called a "recurrence" or "outbreak." Herpes does not always recur, and if it does recur, the timing and severity are different from person to person. Some people rarely have recurrences. Others have them often. Herpes is most likely to recur in the first year after infection. Recurrences may be more frequent for people with weakened immune systems.

^ Nasser M, Fedorowicz Z, Khoshnevisan MH, Shahiri Tabarestani M (October 2008). "Acyclovir for treating primary herpetic gingivostomatitis". The Cochrane Database of Systematic Reviews (4): CD006700. doi:10.1002/14651858.CD006700.pub2. PMID 18843726. (Retracted, see doi:10.1002/14651858.cd006700.pub3. If this is an intentional citation to a retracted paper, please replace {{Retracted}} with {{Retracted|intentional=yes}}.)
^ Xu, Fujie; Fujie Xu; Maya R. Sternberg; Benny J. Kottiri; Geraldine M. McQuillan; Francis K. Lee; Andre J. Nahmias; Stuart M. Berman; Lauri E. Markowitz (2006-10-23). "Trends in Herpes Simplex Virus Type 1 and Type 2 Seroprevalence in the United States". JAMA. 296 (8): 964–73. doi:10.1001/jama.296.8.964. PMID 16926356. Archived from the original on 2010-04-24.
A doctor will base a presumptive diagnosis on information provided by the patient and on the physical examination. The characteristic appearance of the herpes sores leaves little doubt about the diagnosis, so the typical appearance of the sores is key to the diagnosis. This appearance helps distinguish oral herpes from oral thrush, shingles, gonorrhea, and syphilis. In addition, chapped or sunburned lips can resemble oral herpes, but the tissue stain (Tzanck smear, see below) shows no virus-induced cell changes. Further testing is usually not necessary but is sometimes done.
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