Certain maladies which are clue to the action of virulent micro-organisms, and also those in which, up to the present time, no special microbe has been detected, are distinguished from ordinary non-infectious diseases by certain features. The most prominent character of all infectious maladies, which has been recognized from the earliest times, is their tendency to render the system of the animal which they attack proof against a recurrence of the same affection for a considerable period, and in some cases for the life of the subject. Further, they are marked by the occurrence of certain phases or stages, beginning with what is called the period of incubation, by which is meant the time which elapses from the moment of infection up to the declaration of the disease, indicated by the appearance of the first definite symptoms. Following this, which may be described as the period of invasion, there is in eruptive affections a perfectly well-defined series of changes in the character of the eruption, from its first appearance to its decline. Small-pox furnishes the most typical instance of these changes, which begin with the appearance of papules or pimples (papulation). The effusion of serous fluid beneath the cuticle converts the red pimples into vesicles (vesication), subsequently the contents of the vesicles become purulent (pustulation), and at last dry up and form a scab (desquamation), which ultimately falls off, leaving an eschar behind it (pitting.) These changes occur at intervals on an average of something like three days.

In infectious diseases which are not eruptive the stages are not so definitely marked, but there is always the period of incubation, followed by invasion indicated by febrile symptoms, then the progress to the acute stage, and the gradual subsidence of the disorder to the period of convalescence, or, on the other hand, the increase of the energy of the attack until a fatal result occurs.

Another peculiarity of infectious diseases is their refractory character in reference to remedial measures. Medicines are for the most part of little or no value in controlling the progress of these disorders, and it is recognized as an axiom in the treatment of all such affections that the aim of the physician should be to maintain the patient's strength to enable him to resist the effects of the malady during its progress, which it is admitted cannot be arrested.

The recognition of the fact that an attack of an infectious malady exhausts for a certain period the susceptibility of the system, very early led to the adoption of inoculation as a means of controlling the virulence of the disorder, and, further, of causing the attack at a period when it might be considered to be of least importance. When the inoculation was properly performed, even in a virulent disease like small-pox, the resulting attack was generally very mild in its character. The extremely minute quantity of the virus which was employed had a great deal to do with the more benign character of the infection, and the operation had further the advantage of enabling the operator to determine when the disease should be produced, and selecting a period when the patient was in the most favourable condition. The one insurmountable objection which presents itself, both in regard to man and the lower animals, is the danger of communication of the inoculated disease to susceptible subjects, who are as likely to suffer from a severe or fatal attack as if they had taken the affection from the most virulent case. The discovery of certain microbes which were proved to be the cause of disease, and the results of artificial cultivation in modifying this virulence, turned the attention of pathologists to the subject of protective inoculation by means of the ameliorated virus, which was found to produce an extremely mild form of the disease and to confer immunity with a very slight amount of risk. This method of protection has been tried with a certain amount of success in anthrax, and in the disease which is known as Blackleg in young cattle. In this country the system was not at first favourably received; accidents occurred among inoculated animals which led to losses as great as would in ordinary seasons occur in the unprotected. Improved methods of preparation of the virus, and simpler means of inoculation, have since been attended with a large measure of success.

For a long period there was only one infectious disease - glanders - to which the horse was believed to be subject, and in regard to this affection its infectious origin was frequently disputed. Of late years the progress of the science of bacteriology has led to the addition of a number of diseases which are classed as infectious, the term being now understood to include disorders the virus of which may, according to some authorities, be generated in the organism instead of being introduced from the outside. This view, however, of auto-infection is not capable of demonstration, because it is impossible to prove that the infection has originated in the organism, in face of the fact that the atmosphere is capable of conveying the spores of bacteria into the animal's body. The following diseases of the horse in the present day, among others, are included in the term contagious, infectious, or epizootic affections: - Anthrax, variola (horse pox), tetanus, pyaemia, malignant oedema, contagious stomatitis, purpura, strangles, influenza, contagious pleuro-pneumonia (which belongs to the influenza group), cerebro-spinal meningitis, tuberculosis, and certain affections of the skin, as mange, ring-worm, and epizootic lymphangitis, depending on the invasion of animal or vegetable parasites.