Page:Popular Science Monthly Volume 83.djvu/588

584 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 attack 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 animal diseases and the channels through which the infecting microorganisms 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 animals 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 diseases from man. This is not because the virus is unable to infect animals, 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 illustrate by rabies. A mad dog may bite several persons, some or all of whom may develop rabies, or hydrophobia, and die. Dogs or other animals 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 tendency 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, experience 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