286 HEALTH AND DISEASE 



upon and destroy them with the greatest eagerness. The 

 latter are those whose cuts heal quickly and who are 

 not troubled with boils and abscesses. 



Immunity. Certain diseases, 1 as smallpox and scar- 

 let fever, never attack the same person twice, no matter 

 how often he may be exposed to them. From this fact, 

 we infer that the antibodies developed during the dis- 

 ease persist throughout his life. Consequently, the mi- 

 crobes of the disease can never again gain a foothold 

 and he is said to be immune to the disease. In smallpox, 

 immunity can also be gained by vaccination with the 

 microbes derived from a modified and much milder form 

 of the disease, cowpox or vaccinia. Such immunity, 

 however, is limited to a period of from six to fifteen 

 years. In such diseases as diphtheria, on the contrary, 

 the immunity acquired by the reaction of the body lasts 

 but a few weeks after the disease has run its course. 



Tuberculosis. In contrast to these diseases in which 

 the body has the power of curing itself, is that most com- 

 mon disease, consumption or tuberculosis. In consump- 

 tion, the growth and multiplication of the microbes are so 

 slow that they do not give off enough toxin to arouse the 

 body to manufacture an adequate supply of antibodies. 

 The white blood corpuscles seem to remain more or less 

 indifferent to the infecting organisms, and as a result 

 they are not destroyed. If, however, the person is in 

 a condition of very robust health or acquires such .health 

 by invigorating out-door life, the body is stimulated to 

 develop antibodies in sufficient amounts and the course 

 of the disease is thereby checked. 



Artificial development of antibodies. By experi- 

 ments upon animals, it has been found possible to de- 



1 The microbes which cause these diseases have not as yet been 

 established beyond question. 



