The relation between the words “infectious” and “contagious” requires explanation. A disease like measles or small-pox, which can be transmitted from person to person, without immediate contact between the two, is termed =infectious=. In these cases the infection is conveyed by mucus expectorated or by dust blown about, or carried in apparel, etc., from the first patient. Such diseases may also, of course, be communicated by direct contact. If direct contact between the sick and well is indispensable for the transmission of a disease it is called =contagious=. There is no such hard line in nature, although some diseases can be more easily communicated than others. The term contagious is usually applied to parasitic diseases like ringworm and scabies, but even these can be communicated by means of infected articles as well as persons. The word contagious should be abandoned for all the acute febrile diseases. The word =infective= is used to include all specific febrile diseases, however spread. This word, therefore, includes not only infectious and contagious diseases, but also diseases spread by =inoculation=, _i.e._ injection of the infection under the skin. Thus malaria is not infectious from patient to patient; but can be inoculated by the mosquito. =Infective Diseases= are either acute or chronic. Of _acute_ infective diseases small-pox and enteric fever are typical examples; of _chronic_, tuberculosis and syphilis. It was formerly supposed that in certain diseases the contagium or infective agent grew in external noxious matter, a _miasm_ being produced; while in other diseases _contagion_ was only produced direct from patient to patient; and others originated in either way. Hence the classification of infective diseases into (_a_) miasmatic, (_b_) contagious, and (_c_) miasmatico-contagious diseases. This classification has now been abandoned. Thus influenza and ague were formerly thought to be miasmatic; but the former is spread by personal infection; the latter by inoculation of the contagium by an infected mosquito. =Bacteriology= has thrown an immense light on the causation of infective diseases. A large number of these have been proved to be caused by bacteria, and by analogy we infer the same thing for many others. =Koch= has laid down the following =postulates= as necessary before it can be stated that a particular disease is directly caused by a given microbe:— (1) The microbe shall be demonstrated in the diseased tissues or blood of man or an animal suffering or dead from the disease.