Under this heading will be considered all diseases of a contagious character, together with those which arise from miasma. A contagious disease is one which is communicated by actual contact of an individual with palpable substances, originating in individuals suffering with the disease. Infectious diseases are those which are propagated by means of impalpable substances carried in the air. Nearly all contagious diseases are also infectious. In most infectious diseases, the morbid parts which give rise to the disease proceed from individuals suffering with contagious maladies. In some cases, however, as in the so-called malarial diseases, such a connection cannot be traced.

The Germ Theory of Disease

The supposed nature of germs has already been considered. In the case of quite a number of the diseases included under this heading, it may be claimed that absolute proof of the existence of microscopical organisms as specific causes of the affections referred to has been obtained through extensive and searching investigations which have been made respecting this subject. In the case of several, while proof is not absolute, the evidence is such as to leave little room for doubt. Recent investigations of the nature and cause of malarial poisoning seem to have shown beyond reasonable doubt that this class of affections depend upon certain low vegetable organisms which are produced in great abundance under conditions known to be favorable to the development of malarial diseases.

Infectious diseases are divided into two classes, acute and chronic. We shall consider both classes together without any other distinction than that of sequence.