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, 194o).
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-sporawere 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).