HISTORICAL INTRODUCTION 



Pasteur, L. Microscopical and cultural demonstration of the existence of 



an air-spora, and the fermentation of urea by a Al/V/'oro^n/^ . . i 



Koch, L. Introduction of pure-culture methods, and demonstration of spore 



production in bacteria. Discovery of cause of anthrax 



Statement of Koch's postulates .... 



Introduction of gelatine to solidify media . 

 Hansen, G. H. A. Discovery of cause of leprosy 

 Neisser, a. Discovery of the Gonococcus . 

 Koch, L. Discovery of the tubercule bacillus 



Discovery of the cholera Vibrio .... 

 LOEFFLER, F. Discovery of bacillus of swine erysipelas 

 NicoLAiER, A. Discovery of the tetanus bacillus . 



KiTSATO, S 



Yersin, a. 

 ivanovski, d. 

 Beijerinck, M. W 



Discovery of the bacillus of plague 



Discovery of filterable viruses in plants 



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1876 

 1878 

 1881 

 1874 



1879 

 1882 

 1883 

 i88s 



1894 



1892 

 i8q8 



The Hygienists and their In\t:stigation of the Air 



While the causes of infectious diseases of man and animals were being 

 unravelled in laboratories and clinics, a series of field investigations into 

 the air-spora was in progress to find whether fluctuations in number and 

 types of microbes present in the atmosphere were connected with out- 

 breaks of such diseases as cholera, t}^phoid, and malaria. 



Salisbury (1866) investigated the air-spora in connexion with malaria 

 in the Ohio and Mississippi Valleys, by exposing sheets of glass above 

 marshy places during the night and examining them microscopically. He 

 observed small, oblong, Palmella-like cells singly or in groups on the 

 upper side of the glass sheets, but never in the droplets which formed on 

 the underside. He believed that these cells were produced from a grey 

 mould growing on the surface of prairie soil, and were in fact its spores 

 which were liberated at night and rose some 30 to 100 ft. in the air, none 

 being present during the daytime. Their liberation could be prevented by 

 covering the ground with a layer of quicklime or straw. 



Some form of the 'aeroconiscope', invented by Maddox (1870, 

 1 871), was in favour with many investigators in this period. The model 

 used by Cunningham (1873) consists of a conical funnel, with the mouth 

 directed into the Avind by a vane, ending in a nozzle behind which is 

 placed a sticky microscope cover-glass on which were impacted dust 

 particles driven into the cone by the wind (Fig. 2). Cunningham's studies 

 were made in two Calcutta gaols where cholera and other fevers were 

 rife, and where medical statistics were available. He sampled for 24-hour 

 periods, and illustrations of representative catches of airborne organisms, 

 mainly fungus spores and pollens, were published in a series of splendid 

 colour plates. He found no correlation between these micro-organisms 

 and the incidence of fevers in the gaols. Moist weather diminished inor- 

 ganic dusts, but it appeared to increase the total number of fungus spores. 



The most intensive sustained analysis of bacteria and moulds in the 

 atmosphere was made in Paris during the last quarter of the nineteenth 



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