Equine Infectious Anemia is a disease that infects the equids (horses, zebras and donkeys). It is caused by a Retroviris lentivirus which is genetically and antigenically related to other lentiviruses like maedi-visna, caprine arthritis-encephalitis virus (CAE), ovine progressive pneumonia virus (OPPV), human immunodeficiency virus (HIV), feline immunodeficiency virus (FIV), bovine immunodeficiency virus, etc.

Since EIA is an infectious disease and there are infected animals with no apparent symptoms, it is necessary to perform a diagnostic assay to detect the infected animals. 

- Foals may have a "positive" EIA test result up to 9 months of age if they receive antibodies from the milk of their mother. That is the reason why they can have a "positive" EIA result and a few months later present a "negative" test result.