The light factor of environment is very important in the life history of micro-organisms. In general, bacteria flourish and wax fat in the absence of light. To many pathogenic micro-organisms light—especially direct sunlight—is fatal. In rare instances, as in the case of Englemann's bacterium photometricum, light increases activity. I know of no pathogenic germs in which the reverse does not prevail in greater or less degree. That temperature is an important factor in micro-organic life is so familiar that I would not allude to it were it not so vital in its relations to the evolution of infection. Bacteria have been found that multiply at 0 C. Certain water varieties spore in water covered with ice and freezing does not kill them, but most soil and water bacteria thrive best at about 20 C. although they begin to multiply at a lower point. Germs have been found which grow only
LYDSTON GF. THE EVOLUTIONARY ASPECT OF INFECTIOUS DISEASES, WITH ESPECIAL REFERENCE TO THE LOCAL VENEREAL DISEASES.. JAMA. 1902;XXXVIII(21):1366–1369. doi:10.1001/jama.1902.62480210024001i
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