BIOLOGICAL THERAPEUTICS 537 



the auti-bodieSj or immune bodies, and the substances which stimulate the 

 body to the production of such immtme bodies are known as anti-gens. 



Infectious Diseases. 



An infectious disease may be defined as any malady caused by the in- 

 troduction into the body of minute pathogenic organisms of the vegetable 

 or animal kingdom, having the power to multiply indefinitely and set free 

 certain noxious substances, which are chiefly responsible for the morbid 

 changes. INIost of tlie contagious and infectious diseases of animals are 

 due to the action of bacteria, examples, tuberculosis, anthrax, blackleg, 

 tetanus, etc. Others, dourine, rabies, are due to minute forms of animal 

 life — protozoa. Another class are caused by fungi of a higher order than 

 bacteria — examples — actincmycosis and aspergillosis. Lastly, we find a 

 group of infections due to unknown causes. Hog cholera, foot and 

 mouth disease, smallpox, etc., belong to this class and we speak of the 

 organisms that cause them as ultra-microscopic or ultra-visible because we 

 have not been able to detect them by the aid of the most powerful micro- 

 scope. They are also known as filtrable viruses, since they pass through 

 porcelain filters through which the ordinary microorganisms will not pass. 



Bacteria may be defined as minute unicellular organisms of plant-like 

 character. They multiply by cell division or by a process known as spore 

 formation. The spore has analogies to the seeds of higher plants and its 

 development usually occurs when the conditions of growth of the bacteria 

 .■,'re unfavorable. It is a great deal more resistant to destructive agents 

 than the bacterium that produces it. 



The protozoa, lowly forms of animal life, liave a more complex struc- 

 ture than the bacteria and their cultivation under artificial conditions is 

 far more difficult than is the case with the bacteria. To this class belong 

 tlie trypanosomes causing mal de caderas, dourine, tick fever and malarial 

 or biliary fever of horses. 



Mention must also be made of the fungi producing mycotic lymphan- 

 gitis and streptothricosis of horses. 



The infective agents that cause disease may gain access to the body 

 in various ways. They may be swallowed with the food or water — exam- 

 ple, anthrax; be inhaled, example, influenza (probably); may infect 

 wounded surfaces, example, tetanus ; may find their way through certain 

 natural openings, as the milk ducts of the teats ; the genital organs or the 

 eye, examples, abortion disease, hog cholera. Lastly, certain of .them may 

 reach the body of the host through introduction by an intermediate bearer, 

 example, the piroplasm of tick fever, which reaches the blood of its bovine 

 host through the bite of the cattle tick. Some infective agents may enter 

 the body by two or more of these channels. Whatever the cause from 

 which they arise, all of. these contagia have quite definite incubative pe- 

 riods, varying with the different organisms. Animals suffering from in- 

 fectious disease are more or less directly a menace to others; they are 

 factories for the production and distribution of disease germs. The infec- 

 tion may be direct — from animal to animal — or indirect, through infec- 

 tions of the feed, pasture, stable, byre, or by intermediate carriers as 

 flies, buzzards, rats, etc., or lastly, as seen above, through introduction 

 into the bodv bv an inbO/g/^eflfebyoM/crliBC^if^erefore evident that in the 



