172 FOREST INFLUENCES. 



yellow fever, and malaria are, accorclinj? to Dr. Petteiikofer " soil dis- 

 eases of miasmatic orioin." In this connection a distribution must be 

 kept in view between those organisms which are disease producers, 

 pathogenic, and those which are more or less harmless parasites. The 

 latter, Saprophytic bacteria, it must be kept in mind, thrive on decom- 

 posing vegetable and animal matter 5 but pathogenic bacteria thrive 

 best on living organisms, although they occur also outside of them. 



The conditions for the favorable development of the pathogenic 

 bacteria Ebermayer discusses at great length. The facts are stated or 

 established by him that the vegetable components of the forest soil 

 contain less nutritive matter (albuminoid, potash, and phosphates and 

 nitrates) for bacteria growth; that the temperature and moisture con- 

 ditions are less favorable; that the sour humus of the forest soil is an- 

 tagonistic to pathogenic bacteria; finally, that so far no pathogenic 

 microbes have ever been found in forest soil, hence this soil may be 

 called hygicnically pure. 



Only when upper soil strata dry out and a wind, forming dust, 

 sweeps over them are microorganisms carried into the air; hence, with 

 less air movement in the forest, we would expect fewer microbes in the 

 forest air. This expectation is realized in the investigation of Sera- 

 flni and Arata, who tabulate their countings of bacteria, divided into 

 three classes— molds, liquefying, and nonliquefying bacteria for 40 suc- 

 cessive days, from May 6 to July 8 — and find, that with one excei»tional 

 day, one or two of these classes were always less numerous in the for- 

 est than on its outskirts and generally from twenty-three to twenty- 

 eight times less. 



With these detail investigations are in accor<l the general observa- 

 tions in India, where villages surrounded by forests are never visited 

 by cholera, and troops are being withdrawn into forest stations in order 

 to arrest the disease which it has l)een found is invited by removal of 

 forests. Extensive moors, which exhibit similar unfavorable soil con- 

 ditions, have been observed to give the same immunity from the disease. 

 With regard to yellow fever the same observations have been made in 

 our own country. 



If it is considered, says Ebermayer, that the coma bacillus which 

 produces cholera makes great demands in its nutrition and belongs to 

 the most sensitive bacteria, especially sensitive against free acid, being 

 destroyed by acid stomach juices, that when dry it dies quickly and is 

 readdy attacked by decay-producing bacteria, that it prospers best in a 

 temperature of 80° F. to 194° F., and ceases to grow when the tempera- 

 ture sinks below 54° F., the protective influence of the forest is readily 

 explained. 



Malaria has been shown by Marchiafava and Celli not to be a 

 bacterial disease, but to be produced by parasitic protozoa, called by 

 these authors '' plasmodia," which are formed in the red blood corpus- 

 cles and when inoculated upon healthy people produce the disease. 



