The Herpesviridae are a large family of DNA viruses, that cause diseases in humans. The family name is derived from the Greek word herpein ("to creep"), referring to the latent, recurring infections typical of this group of viruses. Herpesviruses all share a common structure - all herpes viruses are composed of relatively large ds linear DNA encoding 100-200 genes and all herpes viruses are nuclear-replicating - the viral DNA is transcibed to RNA within the infected cell's nucleus. Infection is initiated when a viral particle contacts a cell. Following binding, the virion is internalized and dismantled, allowing viral DNA to migrate to the cell nucleus, where replication of viral DNA and transcription of viral genes occurs. During symptomatic infection, infected cells transcribe lytic viral genes. In some host cells, a small number of viral genes termed latency associated transcript (LAT) accumulate instead. In this fashion the virus can persist in the cell (and thus the host) indefinitely. While primary infection is often accompanied by a self-limited period of clinical illness, long-term latency is symptom-free. Reactivation of latent viruses has been implicated in a number of diseases. Following activation, transcription from latency-associated LAT to multiple lytic genes lead to enhanced replication and virus production. Clinically, lytic activation is often accompanied by emergence of non-specific symptoms such as low grade fever, headache, sore throat, malaise and rash as well as clinical signs such as swollen or tender lymph nodes and immunological findings such as reduced levels of natural killer cells. In this family, there are eight human herpes-viruses: Herpes Simplex virus type 1, type 2, CMV, EBV and HSV 6, 7 and 8.