Over the past 10 years, the discovery of a new immunodeficiency syndrome has been made in the Arabian horse breed. Designated as a combined immunodeficiency (CID) because affected foals show a deficiency in both cellular and humoral immunity, the syndrome has attracted much interest due to its seriousness (100% mortality in affected foals within 5 months of birth) and the prevalence of the syndrome in the Arabian Breed (according to one study 2.3% of all Arabian foals are affected).