You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is A four-fold increase in the titre obtained in Weil- Felix reaction is diagnostic of - and explain in detail?
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