100 VETERINARY STUDIES 



present day microscopes. Several important diseases like pleuro- 

 pneumonia and hog cholera are caused by filterable viruses. 



Protozoa. — Protozoa, minute one-cell animal microorganisms, 

 are now recognized as the cause of such serious diseases as 

 dourine, a venereal disease of horses, and syphilis and malaria 

 of the human. 



Practical suggestions. — The student should bear clearly in 

 mind that bacteria or germs usually require very favorable con- 

 ditions for existence, and especially, for retaining disease-pro- 

 ducing power. Sunshine and ventilation, vigorous health, and 

 high resisting power of an animal, are unfavorable to the pres- 

 ence and activity of any contagium. Any factor which increases 

 animal resistance or decreases the virulence of an infection, is a 

 protection against infectious disease. 



Disease germs are actual substances, tiny particles of living 

 matter, which may be carried about in any way that any very 

 fine particles of heavy dust may be scattered. 



The beginning of an outbreak may come in two ways: by 

 recent introduction of germs, for example anthrax in northern 

 states ; or it may apparently be caused by increased virulence in 

 germs already present, but not previously capable of producing 

 disease. Such an increase of virulence may be due to surround- 

 ing conditions, which especially favor germ life, or to prelimi- 

 nary passage through an animal of very low resistance. This 

 probably occurs in pneumonia. 



Apparently the number of germs taken into the body makes 

 a difference in the severity of some diseases — hence the necessity 

 of sunshine to reduce virulence and destroy germs and of venti- 

 lation to carry as many germs as possible out of the stable. 



Bearing in mind now what has been said of germs, it is easy 

 to see how glanders, with infection left in feed boxes or water 

 pails, may be spread from horse to horse ; how hog cholera with 

 its infection, especially in the manure or carcass, may be easily 

 and rapidly spread; or how lumpy jaw may be spread by germs 

 in the pus scattered from abscesses. 



Infectious diseases do not "just happen" any more than mus- 

 tard just happens in a wheat field. In both cases alike there 

 must be definite planting of seed and favorable conditions for 

 its growth. Spore formation on the part of certain bacilli, 

 anthrax for example, is much more than a scientific phenome- 

 non. A pasture which remains contaminated with anthrax 



