Members of the herpesvirus family have been identified in more than 80 different animal species. After the primary infection, herpesviruses establish latency in the infected host. Intermittently, the latent genome can become activated, in response to various stimulus (will be discussed in the Latency chapter), to produce infectious virions.
While the herpesviruses share some aspects of replication, natural history and pathogenesis, they also differ extensively in important details. For instance, HSV-1 and HSV-2 infect a wide host-cell range, are recognized for their neurotropic characteristics, multiply efficiently and rapidly destroy the cells that they infect. EBV on the other hand is largely restricted to B lymphocytes but can induce malignant transformation. Herpesviruses also differ with respect to the cells in which they establish latency and in the clinical manifestations of disease with which they are associated.
Herpesviruses from all species have been classified into three groups based upon of tissue tropism, pathogenicity, and behavior. Alpha herpesviruses are usually fast replicating and, in humans, are represented by Herpes Simplex virus-1 and 2, and Varicella Zoster Virus. The slowly replicating beta herpesviruses are represented by Cytomegalovirus and Human Herpesvirus-6 and 7. Gamma herpesviruses are poorly replicating and readily transform cells. The gamma herpesviruses in humans is represented by the Epstein-Barr Virus (EBV), which is a B-cell transforming virus and the causative agent of infectious mononucleosis, and the Human Herpesvirus-8 (HHV-8) or Kaposi's sarcoma associated virus (KSHV) present in Kaposi's sarcoma. In monkeys, at least 20 different herpesviruses have thus far been described. Most of these viruses have their counterpart in the human system with the exception of T-lymphotropic transforming gamma herpesviruses, which are the causative agents of T-cell lymphomas and lymphocytic leukemias in monkeys. This suggest that while three new herpesviruses has been identified in humans in the last ten years, it is possible that more will be identified. The general characteristics of the Human Herpesviruses are shown in this Table.
The ubiquitous occurrences of CMV, EBV, HHV-6, and -7 in humans and because they can be reactivated under various conditions make it difficult to correlate these viruses to specific diseases. However, new diagnostic methods and increased research have already associated HHV-6, and -7 with specific diseases and lead to discovery of new diseases caused by or associated with EBV.