Makgill. — Natures Efforts at Sanitation. 149 



Infection occurs if the germs of disease can — (1) Live and 

 increase in the body ; (2) produce their injurious substances. 

 It is these injurious substances, called " toxins," which pro- 

 duce the symptoms of disease. This we know, because we 

 can grow in the laboratory a culture of a disease germ — say, 

 typhoid — in broth. Then by filtering through porcelain we rid 

 it of the actual bacteria, and yet the symptoms of the disease 

 can be produced if we inject the germ-free broth. But if we 

 make our culture in the living body, as it were, by infecting an 

 animal, we can also demonstrate the formation in the blood of 

 substances which act antagonistically to the germs and their 

 poisons. 



Natural Immunity. 



Immunity, or the power to combat disease, can be divided 

 into two lines of resistance — (1) Natural immunity ; (2) ac- 

 quired. Natural immunity, the first line of resistance, is 

 shown in certain animals which do not take special diseases. 

 Thus cattle cannot be infected with glanders, a disease of 

 the horse. Man does not take rinderpest, and so on. The 

 reason for this immunity is probably merely an extension of 

 the natural resistance all living bodies have against infection. 

 The white corpuscles of the blood are called "phagocytes," 

 because they eat up germs, as we can demonstrate under the 

 microscope. Probably they secrete a sort of poison to germs, 

 killing them first. 



Guinea-pigs do not naturally suffer from typhoid, yet, as I 

 mentioned before, by keeping them artificially in depressing 

 conditions we can lessen their resistance until they can be 

 infected. Certain individuals possess naturally in their blood 

 a strong resistant power to diphtheria, so that their blood will 

 kill such germs when we mix them together, a power not 

 possessed, unfortunately, by all of us. This is an instance 

 of an unusual development of the first line of resistance. 

 Natural immunity is a subject about which we know as yet 

 very little. 



Acquieed Immunity. 



Acquired immunity is the second line, and this is attained 

 — (1) As an after-result of the ordinary course of infectious 

 disease, so that we do not suffer repeated attacks ; (2) by 

 artificially inoculating into the body cultures of the true or 

 allied organisms, which in some way are rendered less viru- 

 lent than normal, as in vaccination ; or, again, by injecting 

 the poisonous products of such organisms in such small doses 

 that they do not injure us. Yet they cause a reaction in the 

 body, leading to immunity from the disease they themselves 

 cause. 



