ETIOLOGY- CAUSES OF DISEASE 223 



found convenient to classify these diseases under the terms endemic, epi- 

 demic, and contagious or infectious. Endemic diseases, or in regard to 

 the lower animals enzootic, affect individuals in certain parts or districts 

 only where the specific cause exists and remains. Certain malarial fevers, 

 for instance, belong to particular parts of the world, and susceptible subjects 

 living in those districts are liable to be attacked repeatedly, but they do 

 not, on removal from the localities, carry the disease with them; the affec- 

 tions, therefore, of this class are clearly not contagious or infectious. 

 Epidemic (epizootic) differ from enzootic maladies as they do not occur 

 persistently in a particular locality, but prevail from time to time, affecting 

 large areas, passing through periods of accession and decline, and reappear- 

 ing at irregular intervals not necessarily in the places in which they 

 previously prevailed. To this class the common disease among horses, 

 influenza, belongs. Outbreaks of the disease occur in various parts of the 

 country at different times, and during its prevalence large numbers of 

 horses are attacked; after an uncertain period the malady gradually ceases, 

 only to recur in the same form, or with certain variations, probably in the 

 following season. The cause of these maladies is not known, no specific 

 virus has been detected, and the views with regard to their contagious or 

 infectious qualities are very conflicting. 



Contagious and infectious Causes.— The two terms contagious and 

 infectious are by advanced pathologists looked upon as interchangeable, but, 

 notwithstanding, they are commonly used with the meanings which were 

 formerly attached to them at the time when the word contagion was 

 accepted as meaning transmission of disease from an affected to a healthy 

 subject by the actual and gross contact of the virulent matter, while 

 infection was held to represent the less obvious mode of transmission 

 through the medium of the atmosphere or by other even less apparent 

 means. The combined signification may now be taken to indicate the 

 propagation of certain maladies through the transmission in any way of the 

 infecting matter of a specific to a healthy and susceptible subject. Con- 

 tagion or infection may be immediate or mediate. In the first case it is 

 necessary that there should be close association between the diseased and 

 the healthy, so that the transmission of infection is direct; while in the 

 other the infective matter must be conveyed by the agency of persons or 

 substances which have been in contact with, or used about, the affected 

 animal. Some of the contagious and infectious causes are still undefined, 

 while others have been demonstrated to be material and recognizable; the 

 cause, for example, of anthrax, glanders, and tuberculosis, foot and mouth 

 disease, pleuro- pneumonia, cattle plague, sheep pox, swine fever is now 

 known to be in each case due to a minute organism belonging to the large 



