The causes of reactivation are uncertain, but several potential triggers have been documented. A 2009 study showed the protein VP16 plays a key role in reactivation of the dormant virus.[71] Changes in the immune system during menstruation may play a role in HSV-1 reactivation.[72][73] Concurrent infections, such as viral upper respiratory tract infection or other febrile diseases, can cause outbreaks. Reactivation due to other infections is the likely source of the historic terms 'cold sore' and 'fever blister'.

Herpes “triggers” (determining exactly what leads to an outbreak) are highly individual, but with time, many people learn to recognize, and sometimes avoid, factors that seem to reactivate HSV in their own bodies. Illness, poor diet, emotional or physical stress, friction in the genital area, prolonged exposure to ultraviolet light (commonly for oral herpes, such as a beach trip or skiing weekend), surgical trauma, and steroidal medication (such as asthma treatment) may trigger a herpes outbreak.
Although the exact cause of Bell's palsy—a type of facial paralysis—is unknown, it may be related to reactivation of HSV-1.[23] This theory has been contested, however, since HSV is detected in large numbers of individuals having never experienced facial paralysis, and higher levels of antibodies for HSV are not found in HSV-infected individuals with Bell's palsy compared to those without.[24] Antivirals may improve the condition slightly when used together with corticosteroids in those with severe disease.[25]
Prodrome: Early in the phase of reactivation (also called an outbreak), many people experience an itching, tingling, or painful feeling in the area where their recurrent lesions will develop. This sort of warning symptom – called a “prodrome” – often comes a day or two before lesions appear. To be on the safe side, it’s best to assume virus is active (and, therefore, can be spread through direct skin-to-skin contact) during these times.
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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