BACTERIA IN DISEASE 33 



Second: The germ from the affected animal must be 

 grown in pure culture on artificial media. 



Third: Such culture should produce the disease when 

 introduced into a healthy animal. 



Fourth: The same organism must be recovered 

 from the tissues or secretions of the animal thus in- 

 fected. 



Infectious Diseases. Any disease caused by the 

 growth and multiplication of a micro-organism on the 

 tissues of the animal having the disease is called an in- 

 fectious disease. Not all infections are due to bacteria, 

 since malaria and yellow fever are due to minute animal 

 organisms. 



Contagious Diseases. A disease which is acquired 

 by direct or indirect contact with an individual having 

 the disease is termed a contagious disease. All conta- 

 gious diseases are infectious, but not all infectious dis- 

 eases are contagious. The distinction is more apparent 

 than real and is not important. 



Avenues of Infection. The embryo may be infected, 

 though rarely, through the ovum or the spermatozoa. 

 Infection with syphilis may thus occur. The fetus may 

 be thus infected through the placenta with small-pox, 

 scarlet fever, tuberculosis, etc., when the mother suffers 

 from the disease. Bacteria may gain entrance to the 

 body through the healthy skin in rare instances; usually 

 it is through minute wounds of the skin. The mucous 

 membranes of the mouth, nose, throat, bronchi, ali- 



3 



