Lentiviruses are etiological agents of chronic diseases in animals and acquired immunodeficiency syndrome in humans.

Lentiviruses are species-specific, exogenously transmitted retroviruses that have a unique ability to replicate continuously but at a restricted rate in host tissues. This property is thought to be related to the retroviral nature of the replication process (RNA to DNA to RNA) and to the ability of the viruses to do this in cells of the macrophage lineage. The viral genomes are expressed only in certain populations of macrophages and this is dependent on a number of interactive factors including the genus of the host, the age of the host, maturation/differentiation factors in macrophages, the strain of virus and regulatory factors in the virus and the regulatory factors in the virus and the macrophages. Macrophages permissive for virus replication are found in specific tissues and virus replication in the cells causes development of lesions in the particular tissues. The nature of the lesions varies from virus induced necrosis to immunopathology to possible toxic infects of monokines produced by the infected macrophages. Cats and primates have further complicating diseases caused by the remarkable sensitivity of their helper T lymphocytes to infection with their lentiviruses. Elimination of these cells leads to onset of various local and systemic diseases caused by opportunistic agents. Whereas equidae and small ruminant animals develop diseases related to infection in macrophage populations, felines, macaques and humans develop diseases related to both infection in their macrophages and elimination of their T lymphocytes.