The two terms contagious and infectious are by advanced pathologists looked upon as interchangeable, but, notwithstanding, they are commonly used with the meanings which were formerly attached to them at the time when the word contagion was accepted as meaning transmission of disease from an affected to a healthy subject by the actual and gross contact of the virulent matter, while infection was held to represent the less obvious mode of transmission through the medium of the atmosphere or by other even less apparent means. The combined signification may now be taken to indicate the propagation of certain maladies through the transmission in any way of the infecting matter of a specific to a healthy and susceptible subject. Contagion or infection may be immediate or mediate. In the first case it is necessary that there should be close association between the diseased and the healthy, so that the transmission of infection is direct; while in the other the infective matter must be conveyed by the agency of persons or substances which have been in contact with, or used about, the affected animal. Some of the contagious and infectious causes are still undefined, while others have been demonstrated to be material and recognizable; the cause, for example, of anthrax, glanders, and tuberculosis, foot and mouth disease, pleuro-pneumonia, cattle plague, sheep pox, swine fever is now known to be in each case due to a minute organism belonging to the large class of fungi. In the horse there are only three or four diseases which have been shown to depend upon a disease-producing microscopic organism, which in each case is distinguished by certain peculiarities of form and modes of growth. The diseases are anthrax, glanders, and tuberculosis. Strangles is the consequence of the introduction into the system of a pus (matter) producing organism belonging to the streptococci. On the basis of the fact that some contagious maladies depend for their existence on a living organism, the pure cultivation of which outside the body will produce the disease when inoculated into a susceptible subject, it has been assumed that all contagious diseases depend on the presence of similar living beings. This, however, remains to be proved with regard to a considerable proportion of contagious maladies. Small-pox, scarlatina, rabies, for example, and vaccinia, have not up to the present time furnished characteristic microbes, although the search for them has been pursued for a long time past, and is still being carried on with the utmost diligence by experts in different parts of the world.