herpesvirus, any virus belonging to the family Herpesviridae. These viruses are pathogenic (disease-causing) in a wide variety of animals, causing disease in humans, monkeys, birds, frogs, and fish.

The herpesviruses are characterized structurally by virions (virus particles) measuring approximately 150–200 nm (1 nm = 10⁻⁹ metre) in diameter. The outer surface of the capsid (the protein shell surrounding the viral nucleic acids) is icosahedral and is composed of 162 capsomeres (capsid subunits). The capsid itself is surrounded by a lipid envelope containing glycoprotein spikes. Herpesviruses have genomes consisting of linear double-stranded DNA, which is integrated into the host cell chromosome upon infection.

There are three known subfamilies of herpesviruses: Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae. Alphaherpesvirinae contains the human herpes simplex viruses types 1 and 2, which are grouped in the genus Simplexvirus, along with bovine mamillitis virus and spider monkey herpesvirus. Other genera in the subfamily include Varicellovirus, which contains pseudorabies virus, equine herpesvirus, and varicella-zoster virus (the causative agent of chickenpox); Mardivirus, which contains Marek’s disease viruses types 1 and 2 of chickens and turkey herpesvirus; and Iltovirus, which contains gallid herpesvirus 1. The alphaherpesviruses are distinguished from viruses of the other subfamilies by their fast rate of replication.

Betaherpesvirinae, members of which are noted for their relatively slow replication cycles, contains human, rhesus monkey, African green monkey, and chimpanzee cytomegaloviruses (genus Cytomegalovirus). Members of subfamily Gammaherpesvirinae, which is composed of the genera Lymphocryptovirus, Macavirus, Percavirus, and Rhadinovirus, include Epstein-Barr virus, baboon, orangutan, and gorilla herpesviruses, and herpesvirus saimiri. The replication rate of gammaherpesviruses is variable.

Among the best-characterized herpesviruses are those affecting humans, namely herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2). Whereas HSV-1 is transmitted orally and is responsible for cold sores and fever blisters, typically occurring around the mouth, HSV-2 is transmitted sexually and is the main cause of the condition known as genital herpes. HSV-1 may also infect the eye, causing corneal ulcers and visual impairment. Both viruses are highly contagious. HSV-2 may be transmitted by individuals who are lifelong carriers but who remain asymptomatic (and may not even know they are infected). HSV-2 infections have also been associated with the development of cervical cancer.