Herpes simplex virus (HSV) types 1 and 2 are members of the Herpesviridae family of viruses which are large DNA viruses. HSV-1 and HSV-2 are also referred to as human herpesvirus types 1 and 2 (HHV-1 and HHV-2) and belong to the Alphaherpesvirinae subfamily that includes human herpesvirus type 3 (HHV-3) also known as varicella-zoster virus.
HSV-1 and HSV-2 (collectively HSV) are similar viruses but are distinct antigenically and biologically. Whereas both viruses infect mucocutaneous tissue, HSV-1 tends to cause infections of the oral mucosa (cold sores) and HSV-2 causes infections of the genital tract (genital herpes). However, there is an overlap. The typical lesions caused by these viruses are vesicles (sores with clear-appearing fluid) of the mouth, genitalia or skin. Both viruses are transmitted by intimate contact with oral and genital secretions including sexual contact. Like all herpesviruses, HSV-1 and HSV-2 can become latent in an infected person and remain present in the body throughout the remainder of life. HSV-1 and HSV-2 establish latency in the neurons that innervate infected tissue. For HSV-1 this involves the cranial nerves, particularly the trigeminal ganglia. For HSV-2 this involves the sacral nerves. The virus remains in a dormant state during latency but can reactivate following periods of stress and cause lesions near the site of the original infection.
HSV can cause severe infections of the nervous system including meningitis and encephalitis. Infection of immunocompromised patients, such as HIV-infected persons and bone marrow or organ transplant recipients, can be very serious, including severe and life-threatening pneumonia. HSV can also reactivate from latency in the immunocompromised and cause severe generalized infections.