12 A SPECIFIC INFECTIOUS DISEASE 



tious or epizootic disease. It is very important not to mis- 

 take for an infectious disease some form of bod\' disturbance 

 due to a local cause or condition. Animals often suffer from 

 improper food and the conditions of life under which they are 

 compelled to live. It frequently happens that as all ot the 

 animals in a given herd are subjected to like conditions, a 

 number of them, perhaps all, will manifest very similar symp- 

 toms and more or less of them die. Such an occurrence often 

 gives rise to the supposition that the cause of death is some 

 form of infection. Deaths from such causes or under such 

 conditions should be carefully distinguished from an epizootic. 

 In differentiating a non-infectious disorder from a specific dis- 

 ease, it is important, and usually sufficient, to take into 

 account the appended characteristics of an infectious disease. 



1. Cause. An infectious disease is caused by a specific 

 agent. This necessitates as the first requisite an exposure to 

 and an infection with the specific organism. 



2. Period of incubation. The infection must be followed 

 by a certain period of incubation before the development of 

 symptoms. This is the time necessary for the invading micro- 

 organism to become established in the body and to bring about 

 the first symptoms of the disea.se. The incubation period var- 

 ies in diff"erent diseases, and to a certain degree in the same 

 disease, according to the mode of infection and the resistance 

 of the individual. Usually the incubation period of a given 

 disease is practically the same for all individuals of the same 

 species when subjected to the same mode of infection. Ex- 

 ceptions, however, are not rare. 



3. Lesions. The morbid anatomy of an infectious dis- 

 ease is usually nearly the same in animals suffering in the 

 same outbreak, especially when they were infected at or about 

 the same time. It is more common for only a few individuals in 

 a herd to be infected in the beginning and from these first cases 

 for other animals to contract the disease. In many epizootics, 

 the disease appears in an acute form in the first animals 

 attacked while those infected later in the course of the outbreak 

 suffer from a chronic form of the affection. In other outbreaks 



