592 Introduction to the Study of Science 



be susceptible in varying degrees to one kind of infection or 

 another. Susceptibility means lack of resistance to disease 

 microbes and their poisonous products. The conditions deter- 

 mining susceptibility correspond to those cited for immunity, 

 namely, racial, family, and individual. One race may be ex- 

 tremely susceptible to tuberculosis or smallpox, but quite re- 

 sistant to yellow fever or scarlatina. Differences in family 

 susceptibility are commonly assumed, as when it is said that 

 scarlet fever or tuberculosis seems to run in this family or that. 

 Individual members of a family vary remarkably, however, 

 as one member, though exposed in all possible ways, may re- 

 main immune, while all others of the family are afflicted with 

 the disease. Diet, personal hjrgiene, and previous illness are 

 important factors in determining susceptibility, as well as 

 immunity. 



Conditions favoring resistance. Certain familiar, but often 

 neglected conditions will, if faithfully maintained, contribute 

 greatly to natural resistance to pathogenic microbes and their 

 toxins. The chief of these conditions is to keep the body and 

 mind strong and vigorous through a proper distribution of work, 

 play, exercise, rest, and sleep, through a sane diet, and habitual 

 cleanliness. Health gained and sustained through the care 

 and use of all the bodily and mental powers is a prophylactic 

 difficult to equal by any artificial means. 



Why an invading germ may not cause disease. The infective 

 organism is very much like a seed which depends for its growth 

 upon the condition of the soil upon which it falls. If the soil 

 is favorable, having sufficient food materials, moisture, a suita- 

 ble temperature, air, and so on, the seed will develop, mature, 

 and bear fruit. So it is with the infective germ. If the body 

 into which it is introduced is susceptible, that is, affords suitable 

 conditions, as adequate moisture, temperature, mucus, and cir- 

 culatory fluids, the germ will develop, multiply, and overwhelm 

 the body with its products. There are, however, three possi- 

 bilities of failure for the invading germ. First, it may find 



