Herpes simplex virus type 1 (HSV-1), also known as herpes labialis, is the etiologic agent of vesicular lesions of the oral mucosa commonly referred to as "cold sores." HSV-1 can also cause clinical disease in a wide variety of other anatomic locations including the genitalia, liver, lung, eye, and central nervous system. These infections can be severe, particularly in the setting of immunosuppression.
The major clinical manifestations of HSV-1 infection and the methods used to establish the diagnosis will be reviewed here. The manifestations depend upon the anatomic site involved and whether the clinical episode is due to primary infection or reactivation disease. The treatment of HSV-1 related disorders is discussed separately. (See "Treatment of herpes simplex virus type 1 infection in immunocompetent patients".)
The clinical manifestations and treatment of HSV-2 infection are discussed elsewhere. (See "Epidemiology, clinical manifestations, and diagnosis of genital herpes simplex virus infection" and "Treatment of genital herpes simplex virus infection".)
Inoculation of herpes simplex virus (HSV)-1 at mucosal surfaces or skin sites permits entry of the virus into the epidermis, the dermis, and eventually to sensory and autonomic nerve endings. The onset of clinical illness is usually sudden, with the appearance of multiple characteristic vesicular lesions superimposed upon an inflammatory, erythematous base. Primary infection may also be associated with systemic symptoms, such as fever and malaise. In general, the severity of symptoms and the number of lesions is considerably less with reactivation. (See 'Recurrent infection' below.)
The lesions can be painful and last for 10 to 14 days. Vesicles are usually grouped in a single anatomic site; however, autoinoculation of distant locations can occur  (see 'Other cutaneous manifestations' below).