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The human immunodeficiency virus (HIV) is a complex retrovirus that infects human cells, especially those bearing a CD4 receptor, for life. It has an elaborate genetic regulatory system that enables it to remain relatively dormant, replicating slowly but steadily, but on appropriate stimulation it is capable of explosive up-regulation, with the consequent release of high numbers of new infectious virions. HIV is cytotoxic to CD4 lymphocytes, eventually eliminating them from the host. The virus's genetic replicator is extremely error prone, constantly changing its structure to improve its infectivity and avoid an effective host immune response.