A four-fold increase in the titre obtained in Weil- Felix reaction is diagnostic of -: (A) Rickettsial infection, (B) Fungal infection, (C) Spirochetal infection, (D) Viral infection
Answer is A. In weil-felix test, host antibodies to various rickettsial species cause agglutination of Proteus bacteria because they cross-react with bacterial cell surface antigens. Reference: Harrison20th edition pg 1011