INFECTION is a general term used to designate the production of disease by agents which, penetrating into the body from without, multiply in the body, spreading either locally, or to the tissues generally, by means of the blood. Such infective agents have been proved in many cases to be minute parasites belonging to the vegetable or animal kingdom, and it may be regarded that it is so in all. Infective diseases are such as have the character of infection, and the term is to be distinguished from infectious diseases, as the latter conveys the idea of direct communication of the disease from a person or animal already infected, whereas infective diseases, whilst including infectious diseases, also cover such as may be derived from without, without the intermediation of a living infected person. Thus septic processes, malarial fevers, etc., are infective but not infectious. Contagious has a similar meaning to infectious, but implies an even more immediate contact by means of which the infection is conveyed.

The infective agents in so far as they have been determined belong to the lower members of the vegetable and animal kingdoms, the adjective pathogenic being applied to such members of these classes as are capable of infecting the bodies of animals and producing disease. The specific forms of parasites are described further on, but it may be noted here that, with the exception of a few unimportant fungi, they all belong to the microscopic forms of the vegetable kingdom which are generically called Bacteria, or to microscopic forms of the animal kingdom belonging to the Protozoa. As both forms are microscopic the general term Microbes may be applied, and Pathogenic Microbes will include the greater part of the known infective agents.

From what has been stated in the previous section, it will be understood that the infective agents produce their effects by means of poisons or toxines. Probably in all cases the poisons evolved by the microbes produce effects on the structures immediately in contact with them, but in almost all cases the toxines do not limit themselves to the locality, but extend into the blood. Moreover, there are some forms of disease in which the microbe propagates in the blood itself and the toxines are primarily present there. We recognize, therefore, a local and a general action, and where these co-exist in the same form of disease they may be of varying intensity in relation to one another. In these respects we are able to recognize several different categories.

(A) Local Infection

The most important local effects are inflammation, sometimes associated with necrosis. That is to say, the toxine may kill the tissue; but short of that, and, it may be, around the dead structure, there is inflammation. In almost all cases the toxines, although their action may be mainly local, are to some extent diffused outwards, and pass into the blood so as to produce general effects.

(B) General Toxic Effects

The toxines, as just noted, are liable to pass from the infected locality, and produce general effects by entering the blood. It is here a question not of microbes passing into the blood, but of their toxic products; it is a poisoning of the blood, not an infection.

The general toxic effects vary according to the nature of the poison. In most cases fever is produced, as in tuberculosis, septic infection, diphtheria, etc. In others there are more specific effects, such as muscular spasms in tetanus and hydrophobia, soporific conditions in cholera, etc.

In some forms the poison given off is of extraordinary intensity, and the disease is dangerous to life chiefly or entirely from the general toxic effects. The most striking example of this is tetanus, where the microbe propagates locally with quite insignificant effects, but gives off a toxine of almost incredible virulence. Another instance is afforded by diphtheria, in which, although there are important local phenomena, the general toxic influence is especially dangerous. Diseases in which, as in tetanus and diphtheria, there is a specific general poisoning are sometimes called toxic diseases. In these and other cases the toxines may be elaborated by the agents when growing in artificial cultures, and the general symptoms of the disease may be produced by the introduction of the toxines separated from the microbes by filtration or otherwise.

As already pointed out, auto-intoxication may result from the absorption of poisons from agents which are not properly infective. Putrid matters after separation of the microbes causing putrescence produce symptoms when injected into the bodies of animals. Foods which have undergone certain fermentative changes of unknown character may acquire poisonous properties, and their decomposition in the intestinal canal may evolve products whose absorption produces symptoms.

(C) Infection Of The Blood. Septicaemia And Pyaemia

In contrast with the conditions mentioned above, in which only the products of the infective agents pass into the blood, we have cases in which the agents themselves propagate in the blood. To this class of diseases, characterized by blood-infection in addition to a poisoning of the blood by toxines (toxaemia), the terms Septicaemia and Pyaemia are applied. The former term indicates a condition in which the blood is invaded by organisms which propagate actively within it, but which is unaccompanied by suppurative manifestations in the various organs. Such a condition is well exemplified in guinea pigs which have been inoculated with anthrax, or in the naturally occurring disease ("Splenic Fever"), in other animals.

In pyaemia, on the other hand, the presence of organisms in the circulating blood is necessarily accompanied by local suppurative effects in the form of metastatic abscesses. It will be apparent that, in the latter condition, the presence in the blood of pyogenic organisms is implied.