240 AGRICULTURAL BACTERIOLOGY 



professional aid he may employ, it is desirable that every 

 stockman be acquainted with the salient facts concerning 

 the more important of the transmissible diseases of domestic 

 animals, just as everyone should know something of the im- 

 portant transmissible diseases of man, so that he may intel- 

 ligently protect himself from them. 



Infection. In order to produce disease the organism 

 must invade the body, must grow therein, and its by-pro- 

 ducts must exert an injurious effect on the body tissues. 

 This sequence of events is known as infection. The severity 

 of the attack of any communicable disease may vary from 

 one animal to another, owing to the difference in resistance 

 of the host and to a difference in the virulence of the or- 

 ganism, which may be defined as the power of the organism 

 to multiply within the body and produce disease. Little 

 is known of the conditions that increase or diminish the 

 virulence of organisms in nature. The resistance of the 

 host may be impaired by any condition that tends to weaken 

 the body, such as fatigue, exposure to cold, heat, or damp- 

 ness, improper diet, thirst, age, wounds, and other dis- 

 eases. Neither the invading organism nor the host are 

 to be considered as passive agents. The relation between 

 them is a true struggle, a fight to the finish. In the strug- 

 gle the host seeks to overcome the parasite by means that 

 will be discussed later, and the organism protects itself in 

 the progress of its growth and development. In so doing 

 the perpetuation of the species is accomplished. 



The portals of entry into the body are the broken skin, 

 or an injured mucous membrane, the alimentary tract, the 

 respiratory tract, the genital tract, and the conjunctiva, 

 or the mucous membrane of the eye. Many organisms have 

 specific, definite methods by which they enter the body of 

 the host ; for example, the hog cholera organism enters by 

 way of the alimentary tract, while the tubercle bacillus 



