It may be inferred from what has gone before that the diseases grouped under this designation are due to specific infective agents, but the precise character of these agents remains in many cases as yet undetermined. The acute specific fevers are, for the most part, infectious. They also, in the great majority of cases, present a distinct periodicity, that is to say, they tend to come to a spontaneous termination at the expiry of a certain number of days or hours. This, as we have seen, is in most cases to be referred to the production in the course of the disease of an antitoxine which by counteracting the toxine brings the disease to a termination. But this explanation does not apply to the periodicity of the malarial fevers, in which the life history of the parasite determines the peculiar periodicity of the attacks, and it may be so in regard to some other forms. With these exceptions we may associate the fact that, in most of the forms of disease under consideration, immunity is acquired by a single attack of the disease.

In regard to the specific fevers the relation of the infective agents to the phenomena of the disease is various. As the effects in all the forms have the common feature of fever, it may be inferred that the blood is the vehicle by which the agent which directly produces the symptoms is conveyed. But as the actual symptoms are those of intoxication, it may be inferred that the infective agent, the parasite, need not itself be present in the blood, but that it is sufficient if the toxines produced reach that vehicle. Hence we may distinguish two categories, although even here the distinction is not an absolute one. There are diseases of this group in which the infective agent is present in the blood, and there are others in which the infection is localized, and only the toxines reach the blood. Even with this division a further distinction has to be made, as the infective agent may at a certain stage be present in the blood, or may be conveyed by it, whilst it may subsequently be deposited locally and produce an intoxication from its more local seat.

Keeping these facts in view, the principal specific febrile diseases may be briefly reviewed, leaving, however, out of account those whose cause has been definitely determined, and whose pathology will be fitly considered in the section on Bacteriology and Animal Parasites.