which are contagious; while malarial fevers, typhoid fever, yellow fever, cholera, pneumonia, peritonitis, etc., are infectious diseases which are not contagious—at least, they are only contagious under very exceptional circumstances, and those in close communication with the sick as nurses, etc., do not contract these diseases as a result of such close association or contact.
The generalization that all infectious diseases are due to the introduction into the bodies of susceptible individuals of living germs capable of reproduction is based upon exact knowledge, gained chiefly during the past twenty years, as regards the specific infectious agents or germs of a considerable number of the diseases of this class. In some infectious diseases, however, no such positive demonstration has yet been made.
The investigations which have been made justify the statement that each infectious disease is due to a specific—i. e., distinct—micro-organism. There are, however, certain infectious diseases which physicians formerly supposed to be distinct, and to which specific names are given which are now known to be due to one and the same infectious agent or germ. Thus puerperal fever and erysipelas are now recognized as being caused by the same germ, the germ which is the usual cause of pneumonia is also the cause of a considerable proportion of the cases of cerebro-spinal meningitis, etc.
In considering the geographic distribution of infectious diseases we will find it necessary to divide them into two groups, one in which the specific infectious agent or germ multiplies only within the bodies of infected individuals, the other in which it also multiplies external to the bodies of infected individuals when conditions as to temperature, moisture, and organic pabulum are favorable for such external multiplication.
In the first group we have all those diseases which are transmitted only by personal contagion, direct or indirect—i. e., by contact with the sick or with articles infected by such contact (fomites). This list includes smallpox, chicken pox, measles, scarlet fever, mumps, whooping-cough, influenza, and diphtheria.
In the second group we have cholera, typhoid fever, yellow fever, and the malarial fevers.
It is evident that the geographic distribution of diseases of the first group will depend chiefly upon conditions relating to the susceptibility of different races of mankind, their knowledge of preventive measures, such as disinfection, vaccination, and isolation of the sick, their mode of life and intercourse with each other and with peoples occupying different geographic areas, etc. Nomadic savages, or people living upon islands remote from the channels of commerce, are less liable to suffer from infectious diseases of foreign
