The rickettsiae and the diseases they cause have been inten sively studied during the past six years. The recent work was stimulated by the probability that many troops would be exposed to certain of the rickettsial diseases during the war. Another strong stimulus was the development of methods for the culth:ation of large numbers of rickettsiae in the yolk sacs of hens' fertile eggs. Until 1940-41 the investigations of this group or organisms had been primarily concerned with the diseases per se, the epidemi ology, vector transmission, and reservoirs. Zinsser and his group and others had done some work on serological relationships but were handicapped by the fact that production of adequate quanti ties of suspensions rich in rickettsiae was difficult if not almost impossible. Much had been done on the relationships of the rickettsiae to the Proteus X strains although no very satisfactory explanation of the Weil-Felix test had been forth coming. The dif ferentiation of the various rickettsial agents depended almost en tirely upon the disease picture produced in animals and the results of cross-immunity and neutralization tests. These at best were fre quently somewhat crude and demonstrated only major differences. Only Rocky Mountain spotted fever vaccine had been produced in sufficient quantity to be of practical significance. It was derived from the tissues of infected ticks and its manufacture so tedious and costly that the supply was very limited. Many had described methods for the preparation of vaccines against typhus fever but none of these had been definitely proven to be of value, or they involved such difficult and delicate procedures that they were not considered practical.This, then, was the status of the rickettsial field when several methods were described for the cultivation of large numbers of rickettsiae, and the war focused attention upon this group of dis eases. 333 Annu. Rev. Microbiol. 1947.1:333-350. Downloaded from www.annualreviews.org Access provided by University of Bath on 02/04/15. For personal use only. Quick links to online content Further ANNUAL REVIEWS