584 THE POPULAR SCIENCE MONTHLY 



regulations against the use of meat from animals affected with certain 

 diseases were based on the observation that such carcasses were injurious 

 to the consumer or on a religious rite of a somewhat indefinite origin. 

 From the earliest times certain disorders of the human family have 

 been attributed with more or less evidence to infection from animals. 



The discoveries of recent years have shown that the specific cause 

 or microorganisms that produce certain transmissible diseases will at- 

 tack both man and one or more species of lower animals. The most 

 common of these microorganisms are those of glanders, rabies and 

 tuberculosis, although anthrax, cowpox and foot-and-mouth diseases 

 are not infrequently transmitted directly from cattle to man. The 

 studies that have been made concerning the identity of human and ani- 

 mal diseases and the channels through which the infecting microorgan- 

 isms pass from one species to another have indicated very clearly the 

 limitations of the intercommunicability of disease between man and 

 animals. It has been shown that the channels of infection render it 

 relatively easy for the virus of a few diseases to pass from infected ani- 

 mals to their attendants, but that the reverse is not necessarily true. 

 Thus, there are reported many cases of anthrax, glanders and rabies in 

 man that were caused by direct infection from diseased animals. The 

 cases are very rare where animals have been infected with these dis- 

 eases from man. This is not because the virus is unable to infect ani- 

 mals, but because from the nature of things the opportunity for it to 

 pass from infected people to animals does not usually exist. There 

 seems to be a popular misunderstanding on this subject. Let me illus- 

 trate by rabies. A mad dog may bite several persons, some or all of 

 whom may develop rabies, or hydrophobia, and die. Dogs or other ani- 

 mals are not infected in turn by rabid people. If, however, animals 

 should be properly inoculated with the brain of a person who had died 

 of rabies they would develop the disease and die. The natural method 

 of infection in rabies is through the bite of the infected individual. 

 An instinct of the dog is to bite and when rabid this natural tend- 

 ency is accentuated, and consequently many dogs, other animals 

 and people may become infected. Biting is not a dominant instinct of 

 man and consequently the rabid person is not liable to bite dogs or 

 other animals. More than this, his environment prevents him from 

 doing so. While the possibility of transmitting the disease exists, ex- 

 perience shows that animals are rarely, if ever, infected with rabies 

 from man. In like manner, anthrax and glanders may be transmitted 

 from animals to their attendants or to those who may examine their 

 dead bodies, but the infected man does not usually come into contact 

 with the animals in such a way that the virus can escape to them. It 

 is natural that man should attend his animals when they are sick, but 

 when he himself is afflicted he does not, as a rule, look after his flocks. 

 In former times there were large numbers of people infected with 



