[Epidemiology of the plague. Changes in the concept in research of infection chains since the discovery of the plague pathogen in 1894].

Three major plague epidemics have been recorded worldwide up to this day: the Justinian plague in the 6th century, the Black Death in the 14th century and the recent 20th century pandemic. The latter occurred at a time of advanced microbiological knowledge which permitted the etiology and the modes of transmission and spread of this bacterial infectious disease to be clarified. The present thesis is an attempt to describe the changes in plague research that occurred during that period of time. While the German, Austrian, British, Russian and Egyptian plague Commissions studying the Indian plague outbreak after 1896 contributed only little to the fundamental epidemiological knowledge on plague, several individual researchers succeeded in discovering some of the key facts in the etiology and transmission of the disease. Alexandre Yersin discovered the pathogenic agent of plague (Hongkong 1894), E.H. Hankin, P.L. Simond (Bombay 1898) and J.A. Thompson (Sydney 1900) recognized the role of rat plague, M. Ogata (Formosa 1897) and P.L. Simond (Bombay 1898) observed the transmission of the disease by fleas, and A.W. Bacot and C.J. Martin (1914) described the specific mechanism of transmission of plague. Accordingly, fleas transmit plague from rat to man, the efficiency of the flea as a vector depending on a blocking phenomenon specific of each flea species. The Indian rat flea (Xenopsylla cheopis) has been recognized the most efficient vector. Although the involvement of wild rodents was already known shortly after the turn of this century, the concept of sylvatic plague (the plague of wild rodents) as opposed to murine plague (the plague of commensal rodents) only emerged between 1920 and 1950. It led to taking stock of all hosts and vectors of the disease and thereby defining the natural foci of plague. According to this concept plague is primarily a disease of wild rodents which have been carrying it together with their fleas since ever. As man and urban rats only become infected occasionally, they do not constitute chronic foci. The occurrence of persistent foci rather depends on the interaction of resistant and sensitive wild rodents (burrowing animals) as was discovered by French researchers in the Middle East as late as the 1950s and 1960s. The concept of rodent-dependent transmission was somewhat attenuated by the discovery of direct interhuman transmission by French researchers between 1930 and 1965.(ABSTRACT TRUNCATED AT 400 WORDS)