DISEASE IN GENERAL 193 



into corrosive sublimate by the action of the hydrochloric acid 

 of the gastric juice. 



(3) Animal parasites, such as lice, ticks, mites, and worms, 

 through infestation of the body, may cause disease in various ways, 

 as will be shown in a later chapter. They are usually not given 

 so much consideration by livestock men as the damage caused 

 by them deserves. 



(4) Vegetable parasites that are known to produce disease 

 include certain fungi, molds, rusts, and smuts. They grow 

 most luxuriantly on grain and forage which have been improperly 

 harvested. During wet seasons when conditions are most favor- 

 able for their growth this class of parasites thrives best. The 

 animal diseases most commonly produced by vegetable parasites 

 are lump jaw of cattle and forage poisoning which affects both 

 cattle and horses. 



(5) Bacterial parasites include all other microscopic organisms 

 that are capable of causing infectious diseases. Collectively 

 they are known as pathogenic microorganism or disease-producing 

 germs. They are of various types and habits and cause damage 

 by liberating enzymes, producing toxins, transforming the chem- 

 ical reactions of the surrounding media, or by giving off 

 offensive substances (aggressins) that inhibit the defensive reac- 

 tions of their host. Examples of bacterial toxins that are very 

 poisonous are tetanus toxin, the product of the tetanus bacilli, 

 and diphtheria toxin, which is produced by the germs that cause 

 diphtheria. 



(6) Viruses are living agents capable of causing disease and 

 yet are so extremely minute that even the highest power micro- 

 scope is incapable of revealing them. None has yet been success- 

 fully propagated outside the animal body. 



THE PROPAGATION AND TRANSMISSION OF DISEASE 



Infection is the term used to denote all the changes that result 

 from the entrance, growth, and damage of bacterial parasites, 

 i.e., infectious organisms in the body. The phenomena of in- 

 fection are to be considered as reactions that result from the 

 conflict between the infecting germs and the body defences. 

 It should be clearly understood that only a living thing can infect. 

 Moreover, it must have sufficient vitality to multiply in the 



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