The basis of the test is the presence of antigenic cross-reactivity between "Rickettsia" spp. and certain serotypes of non-motile "Proteus" spp., a phenomenon first published by Edmund Weil and Arthur Felix in 1916. Weil-Felix is a nonspecific agglutination test which detects anti-rickettsial antibodies in patient’s serum. Weil-Felix test is based on cross-reactions which occur between antibodies produced in acute rickettsial infections with antigens of OX (OX 19, OX 2, and OXK) strains of Proteus species. Dilution of patient’s serum are tested against suspensions of the different Proteus strains.