“During the elimination of the more virulent races of microorganisms, there goes on as well a gradual weeding out of the most susceptible hosts. In a state of nature in which medical science plays no part, there must occur a slight rise in the resistance of individuals, due to selection and perhaps acquired immunity, which meets the decline of virulence on the part of microbes until a certain norm or equilibrium between the two has been established. The equilibrium is different for every different species of microorganism, and is disturbed by any changes affecting the condition of the host or the means of transmission of the parasite. One result of the operation of this law is the low mortality of endemic as compared with epidemic diseases. Certain animal diseases while confined to the enzootic territory, cause only occasional, sporadic disease, but as soon as they are carried beyond this territory epizootics of high mortality may result. Climate in some cases enters as an important factor, but the most important, perhaps, is the slight elevation in virulence brought about by a more highly resistant host. The most susceptible animals are weeded out and the rest strengthened by non-fatal attacks. The virulence of the microbe rises slightly to maintain the equilibrium. In passing into a hitherto unmolested territory, the disease rises to the level of an epizootic until an equilibrium has been established. “The same is true of human diseases, among which smallpox is a conspicuous example. The great pandemics of influenza, which seem to travel from east to west every one or two decades, soon give away to sporadic cases, and the careful work of many bacteriologists would indicate that the influenza bacilli found at present have fallen to the level of secondary invaders, and are parasites of the respiratory tract in many affections.” Smith describes his hypothesis that the tendency of microbes in perfecting the parasitic habit is to act solely on the defensive. The aim of microorganisms, if we may speak of such, is to become able to live unharmed on the host. If they kill the host they have lost their home. The biologic tendency would be in this case for diseases which were once acute to become more and more chronic and indolent. The extremely virulent parasite, which kills its host, will die with the host unless it has effected a means of exit before its death and escapes into a new host. For this reason Pasteur failed to exterminate the rabbits of Australia. He believed that with races of the bacillus of rabbit septicemia, which were very virulent, and which destroyed life very quickly, all that would be necessary was to start the disease among the rabbits of Australia and that it would tend to spread and would kill off all of the rabbits. But the parasite killed the animals before it had perfected for itself a means of escape from the body and thereupon died.