Rickettsias are gram negative micro-organisms, coccoid or bacillary in form, found typically in arthropods. Those known to be pathogenic for man occur intracellularly in the tissues of their animal and arthropod hosts. These organisms have never been cultivated on artificial mediums free from living cells, but they grow and multiply in the various tissue culture mediums. With the exception of the rickettsia of Q fever, they do not pass filters that retain pathogenic bacteria. Several species that are not known to be associated with any disease of man or other animal have been described and named, and one species has been described that is pathogenic for lower animals but not, apparently, for man—this is Rickettsia ruminantium, the causative agent of "heart water disease" of sheep, goats and cattle; the identification of these organisms is based on appearance, staining reactions and occurrence in arthropods. Bodies resembling rickettsias have been isolated from