DISEASE, INFECTION, AND RESISTANCE 277 



growing in the body and producing certain diseases might 

 develop there substances not injurious to the health of the 

 individual, but which would prevent further growth of the 

 microorganism in question. Studies of the blood, however, 

 soon revealed this suggestion to be fallacious. Immunity 

 apparently is not due to the retention of noxious substances 

 produced by the microorganisms causing the disease. 



Pythogenic Theory of Disease. It was also suggested 

 that filth and decaying organic material might be the cause 

 of disease. A half century ago it was believed demon- 

 strably true that malaria was due to the inhalation of the 

 miasms arising from the decomposition of organic matter in 

 swamps. Murchison, in England, contended that filth and 

 accumulated dirt, particularly decaying organic matter, 

 produced many types of fever. When the germ theory of 

 disease was suggested, the pythogenic theory was modified 

 and it was contended that pathogenic or disease-producing 

 microorganisms might be generated in such decaying heaps 

 of organic substances, gain access to the air, and produce 

 disease as a result of inhalation. It was even contended that 

 disease germs might originate spontaneously in such ma- 

 terial. While we now know that disease may sometimes be 

 spread as a result of the accumulation or presence of filth 

 and dirt, nevertheless, it has been quite definitely proved 

 that these factors do not stand in the causal relationship 

 to disease which was suggested by the proponents of the 

 pythogenic theory. 



Ehrlich's Humoral Theory of Immunity. As a result 

 of detailed studies Ehrlich came to the conclusion that 

 immunity is due to the development by the body tissues of 

 substances which will in some way antagonize the invading 

 microorganisms or neutralize their poisonous and deleteri- 

 ous products. The substances thus produced in the tissues 

 he termed antibodies. An individual, therefore, becomes 

 immune or is naturally immune to disease because of the 



