BIOLOGY IN HUMAN AFFAIRS 



with which we are in constant competition. These forms 

 are both animal and vegetable, and they affect health in a 

 great variety of ways. Poisonous snakes and plants, for 

 example, have in some times and places been serious health 

 menaces, and a great deal of study has been necessary in 

 order to develop protective measures. With the advance 

 of civilization, however, the emphasis has changed from 

 these external and obvious dangers to the more insidious 

 and difficult problems of parasitism. Parasites are animals 

 or plants, readily visible or microscopic in size, which live 

 in the body at its expense. In many cases they exert little 

 harmful effect beyond diverting a portion of the food supply 

 from the tissues. In others they produce poisons which 

 irritate, disarrange, or even destroy the tissues of the host. 

 The bacteria, for example, which are microscopic plants, 

 are capable, according to their species, of producing such 

 specific diseases as cholera, plague, typhoid fever, dysen- 

 tery, pneumonia, and a host of others. 



There is hardly any fact which can be ascertained by 

 scientific means concerning the myriad forms of parasitic 

 plants and animals which may not find some application 

 in medicine and public health. Physicians and health 

 officials, recognizing the importance of such knowledge 

 to their specialties, have therefore supplemented the studies 

 of zoologists and botanists by researches of their own, 

 and in many branches have well repaid, by their contri- 

 butions, the great debt which they owed to the biological 

 sciences. For example, they have virtually created the 

 subject of medical bacteriology, have made important 

 contributions to entomology, and may safely claim as their 

 own the sciences of serology and immunology. 



The dependence of medicine and public health upon the 

 biological sciences is so great that it is futile to 

 attempt a list of even the special subjects concerned. 

 The case may be briefed by the statement that without 



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