H AIDS TO BACTERIOLOGY 



acquirement of the disease, while if the contaminating 

 matter was conveyed aerially it was known as infection. 

 At the present time distal aerial convection is seriously 

 entertained only in regard to smallpox, and even over 

 this disease opinions differ. Now, no distinction is made 

 between infection and contagion, and it is usual to class 

 diseases associated with micro-organisms as ' infective.' 

 The principal methods of infection are 



1. Pulmonary infection, the bacteria or spores being 

 inspired. Except where the bacteria are protected, as when 

 in moist droplets, infection through the air is not common. 



2. Intestinal infection, the organisms being swallowed 

 with food, water, or dust. 



3. Inoculation through a wounded or unwounded 

 surface of the skin or mucous membrane. 



4. Inoculation through the agency of some biting insect 

 or other intermediate host, a developmental cycle usually, 

 if not always, being passed in the host. 



Infection by contagion, fomites, etc., may also occur, in 

 which the manner of entrance of the virus into the body 

 is not precisely understood. 



Infection may be restricted to a particular portion of 

 the body (local infection), or may be distributed more or 

 less consistently thereover (general infection). Septi- 

 ccpmia is the term applied to the infection when the 

 organism is carried over the body in the blood-stream, 

 saprcemia when the organism is localised and the bacterial 

 products alone enter the system, and pycemia to the 

 development of metastatic abscesses in the liver, joints, 

 lymphatic glands, etc. 



Epidemics. Many diseases are more or less persistent 

 in a restricted locality (endemic). Thus leprosy is endemic 

 in the Sandwich Islands, cholera in the Ganges Delta, and 

 smallpox in the Soudan. On occasion a disease may spread 

 over wide areas, when it is said to be ' epidemic,' or if 

 spreading over the globe, more or less, it is ' pandemic.' 



Susceptibility. The power of an organism to infect is 

 determined by many conditions. Hunger, thirst, ex- 

 cessive fatigue, exposures to extremes of temperature, 

 debility, and immaturity, all predispose an individual to 

 infection. On the part of the organisms many considera- 

 tions are involved. If subcultured through many genera- 

 tions on artificial media, most become ' attenuated ' 



