This means they cannot function independently outside the living cell. Once inside, however, they provide a far different picture. They are parasitic. This means they live off the host at the host’s expense. Unless you have already been exposed to a particular virus, your body is essentially unable temporarily to prevent viral multiplication inside your body.
As of 2017, there is not currently a herpes vaccine available to prevent HSV-1 or HSV-2. (There is a vaccine available for another virus, herpes zoster; however, despite the similar name, it actually refers to the shingles virus. And, in fact, shingles occurs due to the reactivation of yet another virus, varicella zoster, which causes chicken pox.)
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.