On Dec. 23, 1938, Herald R. Cox published: Use of Yolk Sac of Developing Chick Embryo as Medium for Growing Rickettsiae of Rocky Mountain Spotted Fever and Typhus Groups in JSTOR. Cox discovered that rickettsiae could be cultivated successfully in the yolk-sacs of chick embryos. During World War II, all rickettsial vaccines were produced by this method.

There are four Rickettsia species that frequently cause incapacitating, life threatening illness: Rickettsia prowazekii, R. rickettsii, R. conorii, and R. typhi. R. prowazekii was the first biologic weapon developed by the Soviet Union during the 1930s. It was produced in powder and liquid forms for use as an aerosol