270 BIOLOGY OF SPECIALIZED GROUPS 



(4) The Subject Infected. The susceptibility of indi- 

 viduals to infections varies with the (i) species, (2) with race 

 and individual idiosyncracies, and (3) with the age of the indi- 

 vidual. The species of animal which is attacked by a patho- 

 genic microorganism may be resistant (immune) or sus- 

 ceptible. An animal may be naturally immune to infection 

 or may have acquired immunity by having a disease or by 

 vaccination with a pathogenic microorganism in a modified 

 form. For example, animals are immune to such diseases as 

 gonorrhoea, leprosy, etc., which affect man. They do not 

 naturally contract such endotoxic diseases as typhoid fever, 

 Asiatic cholera, but may be artificially infected by the bac- 

 teria when introduced artificially. Tuberculosis is more 

 common among the negro race than among other races. 

 Diminished vitality of the body due to exposure, use of drugs, 

 injury, loss of sleep, overwork, and disobedience of other nor- 

 mal hygienic conditions influence the results of an infection. 

 There are certain diseases, for example, diphtheria, which is 

 more common in younger life than in adult life. Tuberculosis 

 is a disease which is more common in early life (25-30) than 

 at any other period of life. There is evidently more suscep- 

 tibility to infection at certain periods of life. 



In regard to source of the infectious microorganisms, it 

 may be briefly stated that pathogenic bacteria and some 

 other infectious organisms, etc., may be normally present in 

 the nose, mouth, intestines, respiratory tract, on the skin, and 

 may also be introduced through the agency of food and drink. 



