The subject of infection is complicated, and the medical doctrine concerning it is far from certainty despite the multitude of facts presented by bacteriologists, chemists, pathologists, and clinicians. Before the days of bacteriology the term Infectious commonly was applied to diseases produced by no known or definable influence of any person on another, but wherein common climatic or other widespread conditions were thought to be chiefly instrumental in the diffusion. The contagious disease was one transmitted by contact with the patient, either directly by touch, or indirectly through the use of the same articles.
Now we know that many diseases called infectious are caused by micro-organisms, and we group others under this class because we hold theoretically that they have their origin in microbes not yet isolated. Hence we define an infectious disease as one which is caused by a living pathogenic micro-organism, which enters the tissues from without, and is capable of multiplying therein. These micro-organisms have a time of incubation during which a poison is made in the tissues, and this brings about the intoxication we call the disease.
Infection is a general term that includes contagion; and contagious diseases are infective diseases that may be transmitted directly or indirectly from patient to patient.
The pathological micro-organisms with which we shall deal in this article are (1) the Schizomycetes or Fission-Fungi, which are microscopical organisms that multiply by fission, and are commonly known as Bacteria; and (2) a few Protozoa, which are animal micro-organisms.
The bacteria are classed with plants because, like plants, [{169}] they derive nourishment from both organic and inorganic material. They have no seeds or flowers, but many of them are reproduced by spores. They consist of cells, single or grouped, which when spherical are called cocci, when rod-shaped, bacilli, when spiral, spirilla. There are various subdivisions of these groups. We do not know whether bacterial cells have nuclei or not.
A micro-organism is a parasite when it can live in animal tissues. It is a saphrophyte when it can exist outside animal tissues. If a parasite cannot exist outside animal tissues, it is an obligatory parasite; if it can, it is a facultative saphrophyte. Similarly the saphrophytes are classed as obligatory saphrophytes and facultative parasites. Pathological micro-organisms have very complicated products which are in large part poisonous.
Bacteriologists require seven conditions to prove a micro-organism the specific cause of a given disease, and all these conditions have been fulfilled for anthrax, diphtheria, and tetanus. The specificity has been satisfactorily settled for glanders, malaria, tuberculosis, actinomycosis, gonorrhoea, and malignant oedema. It has been practically settled for typhoid, influenza, the Madura disease, and the bubonic plague; and incompletely defined for leprosy, relapsing fever, and Malta fever.
There are certain diseases which are not called specific, because they may be produced by various micro-organisms. These are pneumonia, osteomyelitis, septicaemia, pymaeia, endocarditis, meningitis, erysipelas, angina Ludovici, broncho-pneumonia, and similar maladies. Cholera and dysentery also might be grouped with these, as cholera appears to be produced by various vibrios and dysentery by different amoebae.
There are other infective diseases, in which we have not yet found the causative micro-organism, but we presume its existence. These are: rabies, syphilis, yellow fever, dengue, typhus, mumps, whooping-cough, smallpox, measles, scarlet fever, and others among the exanthemata.
Malaria and similar diseases are caused by plasmodia, which are protozoa and not bacteria.