ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 275 



close air. People who live entirely in the open air and in well-venti- 

 lated, clean places do not suffer from it, except in the few cases where 

 it may be inherited or introduced from without. It is a disease of civ- 

 ilization, and many countries have been unaffected until the virus has 

 been brought by human agency. 



Soil is not concerned in the prevalence of most endemic and epidemic 

 diseases, though many may have originally sprung from the soil, and 

 some have located themselves in certain areas from which they spread 

 over the globe. The small part played by soil emanations in the great 

 majority of spreading diseases is exemplified by the extension of epi- 

 demics and of endemics like consumption, diphtheria, measles, and 

 whooping cough, in countries which are covered with snow and con- 

 gealed with frost. When once introduced they pass among the popula- 

 tion whose habits are favorable to their growth. In islands, again, 

 when an infectious disease, such as measles or influenza, is introduced, 

 it spreads as fast as in countries where the soil might be supposed to 

 nourish the bacillus or micrococcus independently of the human body. 

 On board ships and in isolated institutions where opportunities are 

 given by association, many infectious diseases spread just as they might 

 in inhabited places, whatever the soil. At the same timeendemicity is 

 largely a matter of soil and habitation. Infection from person to per- 

 son, and to a great extent through confined air, may thus be separated 

 off as the main condition of prevalence of infectious diseases. 



Diseases capable of transmission for a short distance through the air 

 may, for present purposes, be divided into the following classes : 



(1) Those which arise from damp soil or subsoil in alluvial plains, 

 deltas, valleys, mangrove swamps, certain sandy coast districts, and 

 other situations. Malaria, intermittent fever, and ague are the chief 

 diseases of this type, and are in general not transmissible from person 

 to person. They are transmissible a few miles through the air from the 

 locality of origin. Colds and sore throats probably arise from similar 

 conditions, and are infectious through a short distance of air to suscep- 

 tible persons. Forms of dysentery and certain diseases of the liver, 

 etc., seem to be due to conditions largely corresponding with those of 

 malaria. 



(2) Diseases which arise in somewhat similar conditions, but seem to 

 have required not merely vegetable matter, but a large population and 

 neglected filth in the soil and water for their development. Cholera 

 belongs to this class, and depends to a great extent on human filth in 

 the soil and befouled water. Cholera is infectious from person to per- 

 son through the air, but only to a slight extent, and depends for its 

 existence beyond its habitat on access to filthy soil, water, or places 

 where it grows, multiplies, and infects the air, as well as other matter, 

 which gains access to the body. Typhoid grows on damp human filth 

 and may infect persons who breathe the air arising from such filth, 

 especially in houses and confined places. The air in the neighborhood 



