Introduction
although it is clear that HIV is the underlying cause of AIDS and AIDS related disease, its origin remains obscure. There is firm serological evidence of infection on the east and west coasts of the USA from the mid 1970s, and HIV infection in central Africa may have antedated infection in North America. Phylogenetic analysis of the HIV-1 genome has suggested an origin in chimpanzees while, in the case of HIV-2, similarity to the simian immunodeficiency virus (SIV) genome may point to an origin in sooty mangabey monkeys. In both cases the butchery and consumption of these “bush meats” has been incriminated in transmissions to the human host. Like some other RNA viruses, HIV appears to have mutated and shifted its host range and virulence, explaining how a new pathogenic retrovirus could arise in man. Its virulence may since have been amplified as a result of travel, population dislocation and promiscuous sexual contact, with rapid passage of the virus. Retroviruses are so named because their genomes encode an unusual enzyme, reverse transcriptase, which allows DNA to be transcribed from RNA. Thus, HIV can make copies of its own genome, as DNA, in host cells such as the human CD4 “helper” lymphocyte. The viral DNA becomes integrated in the lymphocyte genome, and this is the basis for chronic HIV infection. Integration of the HIV genome into host cells is a formidable obstacle to any antiviral treatment that would no just suppress but also eradicate the infection. Nevertheless, modern treatment with combinations of nucleoside analogues and protease inhibitors has transformed the prognosis for carriers of HIV, usually achieving a sustained fall in virus concentration in blood and restoration of the main target cell (CD4 lymphocyte) to near normal levels. By contrast, the inherent variability of the HIV genome and the failure of the human host to produce neutralizing antibodies to the virus, as well as technical difficulties and concerns about safety, have continued to frustrate attempts to make an effective vaccine. This must not, however, allow efforts to develop and evaluate candidate vaccines to slacken. A particular concern is that a useful candidate vaccine (probably a recombinant envelope vaccine developed in North America or Europe against the locally prevalent HIV-1 B subtype) would be ineffective in those parts of the world where other subtypes predominate. WHO estimates that in the year 2000 there are 36 million carriers of HIV worldwide, and only a small fraction of them have access to suppressive treatment. Both their contacts, their dependants and possibly they themselves would have their life prospects transformed by an effective, or even partially effective, vaccine, and successful application of antiviral treatment in developed countries should in no way be allowed to deflect attention from the necessity of developing and delivering an effective vaccine and of promoting “safe sex” behavior.
HIV and related viruses
