Equine infectious anemia (EIA) is a chronic, relapsing infectious disease of horses caused by a non oncogenic retrovirus. Virus persists in infected animals for life and can be reliably detected by serologic tests that measure levels of antibody to the major structural protein of the virus. Periodic virus replication in macrophages leads to an immunologically mediated acute disease characterized primarily by severe anemia. Recrudescence of acute EIA is the result of antigenic variation of the surface glycoprotein of EIA virus. The frequency and severity of clinical episodes of EIA decrease in most horses, leading to an inapparent carrier state. This cessation of clinical illness is probably brought about by the ability of the infected animals to eventually achieve a threshold efficiency of the immune response against antigenic epitopes common to all EIA virus strains.