(_a_) The most common infectious diseases of animals are transmitted more or less directly from other animals of the same species. Human beings get nearly all their diseases from other human beings who are sick; horses, from other horses; cattle, from other cattle; swine, from swine, etc. Occasionally transmission from one species to another occurs. Tuberculosis of swine most frequently results from feeding them milk of tuberculous cattle or from their eating the droppings of such cattle. Human beings occasionally contract anthrax from wool, hair and hides of animals dead of the disease or from postmortems on such animals; glanders from horses; tuberculosis (in children) from tuberculous milk; bubonic plague from rats; rabies practically always from the bites of dogs and other rabid animals, etc. The mode of limiting this class of diseases is evidently to isolate the sick, disinfect their discharges and their _immediate_ surroundings, sterilize such products as must be handled or used, kill lower animals that are dangerous, and disinfect, bury properly, or destroy their carcasses. Classes of the sick that are especially dangerous for the spread of disease are the mild cases and the undetected cases. These individuals do not come under observation and hence not under control. (_b_) This class of carriers offers a difficult problem in the prevention of infectious diseases since they may continue to give off the organisms indefinitely and thus infect others. Typhoid carriers have been known to do so for fifty-five years. Cholera, diphtheria, meningitis and other carriers are well known in human practice. Carriers among animals have not been so frequently demonstrated, but there is every reason for thinking that hog-cholera, distemper, roup, influenza and other carriers are common. Carriers furnish the explanation for many of the so-called “spontaneous” outbreaks of disease among men and animals. It is the general rule that those who are sick cease to carry the organisms on recovery and it is the occasional ones who do not that are the exceptions. In those diseases in which the organism is known it can be determined by examination of the patient or his discharges how long he continues to give off the causative agent. In those in which the cause is unknown (in human beings, the commonest and most easily transmitted diseases, scarlet fever, measles, German measles, mumps, chicken-pox, small-pox, influenza), no such check is possible. It is not known how long such individuals remain carriers. Hence isolation and quarantine of such convalescents is based partly on experience and partly on theory. It is highly probable that in the diseases just mentioned transmission occurs in the _early stages only_, except in small-pox and chicken-pox where the organism seems to be in the pustules and transmission by means of material from these is possible, though only by direct contact with it.