Rhazes (880-932) believed that smallpox was due to a fermentation of the blood which was permitted by its “excess moisture.” Immunity is explained by expulsion of the “excess moisture” from the body during the first attack.

International Union of Immunological SocietiesUniversity of South AfricaInstitute of Infectious Disease and Molecular MedicineElizabeth Glazer Pediatric Aids FoundationStellenbosch University