<prompt> A four-fold increase in the titre obtained in Weil- Felix reaction is diagnostic of -Rickettsial infectionFungal infectionSpirochetal infectionViral infection  <response>: 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 