THE MICROBIOLOGY OF THE ATMOSPHERE 



carried in suspension per cubic metre of air. He then abandoned the 

 method — remarking, however, that it could doubtless be improved and 

 used more extensively to study the effects of seasons and localities, and 

 especially during outbreaks of infectious diseases. 



The Germ Theory of Disease 



We must now look back and trace the growth of the microbial theory 

 of disease, that had been developing for more than a century. 



The minute growths of fungus noticed for centuries on mildewed or 

 'rusted' plants were believed to be a consequence of the diseases; the 

 dusty powder on rusted wheat was regarded as a curiously congealed 

 exudation of the diseased plant itself. But might this not be putting the 

 cart before the horse ? Could the rust possibly be the cause of the disease 

 instead of an effect? Perhaps the first to give reasonably affirmative 

 evidence was Fontana (1767), who examined wheat rust with his micro- 

 scope and described what he saw as a grove of parasitic plants nourishing 

 themselves at the expense of the grain. 



As further crop diseases were studied it became clear that, in some, 

 infection is acquired by planting in contaminated soil, while others are 

 carried on seed and still others are spread in the wind by airborne fungus 

 spores {see Large, 1940). 



The discovery that microbes can cause disease in man and animals 

 came somewhat later, and the first animal pathogens to be recognized 

 were again fungi — no doubt because they were easier to find than bacteria. 

 In 1835, Agostini Bassi showed conclusively, by inoculation experiments, 

 that a specific mould is the cause of the 'muscardine' disease of silkworms 

 which was then threatening the silk industry of Piedmont. Next, histori- 

 cally, came the recognition of the fungi causing favus, ringworm, and 

 'thrush' in man, as a result of the work of David Gruby and Charles Robin. 



Pasteur had demonstrated that microbes are normally abundant in 

 the air. Many of them can cause fermentation or putrefaction when intro- 

 duced into sterile organic substrates; and it was natural to speculate that 

 others might be the causes of epidemics of some of the so-called 'zymotic' 

 diseases whose etiology was then unknown. Medical workers soon began 

 a systematic search among airborne microbes for the unknown causes of 

 infectious diseases. 



The search was long, and on the whole unfruitful because most epi- 

 demic diseases that attacked man were gradually traced to sources other 

 than the outdoor air. However, in the course of the search, most of the im- 

 portant characteristics of the air-spora were discovered — and then forgotten . 

 The search occupied the last thirty years of the nineteenth century and 

 coincided with the golden age of bacteriology. Listing the dates of contem- 

 porary salient advances in bacteriology will help to give the background to 

 this phase of aerobiology {see Bulloch, 1938). 



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