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HerpesvirusesHerpesviruses infect members of all groups of vertebrates, as well as some invertebrates. Herpesviruses have been typically classified into three groups based upon details of tissue tropism, pathogenicity, and behavior under conditions of culture in the laboratory. The same host can be infected with multiple distinct and unique types.
The three types include: the alpha-herpes viruses which are neurotropic, have a rapid replication cycle and (usually) a broad host and cell range; and the beta- and gamma-herpesviruses which differ in genome size and structure but which both replicate more slowly and in a much more restricted range of cells of glandular and/or lymphatic origin. To date, eight discrete human herpesviruses have been described; each causing a characteristic disease.
Herpes simplex virus types 1 and 2 (HSV-1 and -2) are the primary agents of recurrent facial and genital herpetic lesions, respectively. HSV-2 infection can be classified as a sexually transmitted disease (STD), and infections (although mild) can lead to significant psychological trauma. They are also a major cause of encephalitis -- a much rarer but very serious affliction. Varicella zoster virus (VZV) is the causative agent of chicken pox and shingles. The latter is seen mainly in the very old or in immune compromised individuals.
Infections with human cytomegalovirus (HCMV)--the prototype of the beta-herpesviruses--are linked both to a form of infectious mononucleosis, and to congenital infections of the nervous system. CMV infections of the eye (CMV retinitis) is a major complication in late term AIDS mediated by fuminant HIV infection. In contrast, infections with two other lymphotropic herpesviruses, the closely related beta- human herpesviruses -6 and -7 (HHV-6 and -7) are generally mild early childhood diseases. It is not clear whether they have a causative role in any more serious human afflictions.
Infections with two human herpesviruses, the gamma-prototype Epstein-Barr virus (EBV) (which is a causative agent of infectious mononucleosis) and the recently described Kaposi's sarcoma herpesvirus or human herpesvirus-8 (KSHV or HHV-8) are linked to human cancers. In the case of EBV, despite its high frequency in the general population, carcinogenesis is linked to specific environmental agents-notably in tropical Africa and in South-east China. In contrast to the ubiquitous occurrence of HCMV, EBV, HHV-6, and -7; HHV-8 may be associated with populations at high risk for sexually transmitted diseases. There is both epidemiolgical and strong molecular evidence that it plays a causative role in Kaposi's sarcoma. This tumor was formerly found only in people (mainly men) of advanced age, but the prevalence of HIV-induced immunosuppression (AIDS) has lead to a significant increase in its occurrence in younger men.
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