Caprine arthritis encephalitis (CAE) was first recognized in the United States in the 1970's. CAE is a lentiviris. A lentivirus is any of a group of animal viruses that cause diseases having an unusually long incubation period. An infected goat may remain subclinically infected or may develop clinical disease.  Clinically infected goats may have one or more clincal symptoms which may include arthritis, progressive paralysis, neurological dysfunction, induration of the udder (hard udder caused from inflammation, resulting in abnormally low milk secretion) chronic interstitial pneumonia or progressive weight loss.  The definition of a retrovirus is any of a group of viruses, many of which produce tumors, that contain RNA and reverse transcriptase, including the virus that causes AIDS.