SOURCES OF INFECTION i. As a result of the invasion of a population by a given species of pathogenic micro-organisms certain phenomena are produced, some of which are of present interest, particularly those dealing with the incidence or prevalence of disease. This constitutes the field of epidemiology, which may be defined as the science treating of the sources and routes of infection and their activities, as manifested by the incidence or prevalence of an infectious disease in a given population.
2. Before proceeding further certain fundamental definitions had best be presented: (a) An infectious disease is produced as the result of the invasion of the tissues of an animal or plant by a living organism. Because the invading organism may be in many instances transferred from individual to individual of the host species, these diseases are perhaps more aptly termed communicable. Those communicable diseases which are transmitted exclusively or, nearly so, directly from host to host are spoken of as con tagious, or popularly as "catching." (b) Following the invasion of the host by an organism capable of producing disease, a variable period elapses before the char acteristic symptoms are manifested and during which the ordinary health is apparently maintained. This period is the incubation period. With some infections, as small pox and measles, its length is very constant, with others such as typhoid, it is subject to considerable variation. Its duration is probably dependent upon the number and virulence of the invading organisms on the one hand, and to the specific degree of the host's resistance on the other.
(c) A primary case of a communicable disease is the first case arising within a given area, which is the ascribed source 24 of the infective agents transferred to other individuals who later develop the same infection.
A secondary case of a communicable disease arises as a result of infection received from a pre-existing known case, with which it has been associated. Both are relative terms.
A vector is any intermediate object which serves to transfer infective agents from one host to another. The use of this term is commonly restricted to those insects which occupy this position.
An epidemic is the occurrence within a limited time among a limited population, of an unusual number of cases of communi cable disease.
A pandemic is similar to an epidemic, except the population concerned is much larger, that of an entire continent or the world.
Endemic refers to the usual prevalence of a communicable disease among a limited population.
Epizootic is a term analagous to epidemic, referring to the diseases of lower animals.
3. Sources of Infection.—As a result of extensive bacterio logic researches as well as epidemiologic studies of similar magni tude, performed by scientific observers everywhere it may be stated as axiomatic that: All communicable diseases arise from the invasion of a sus ceptible host by organisms derived from a pre-existing invaded host. The pre-existing invaded host is known as the source of infection.
It has long been observed as a fairly constant characteristic that a greater incidence of these diseases was found among in dividuals in contact with those sick with the same disease, although many cases certainly occur that do not present such a history. At one time in the early days of bacteriologic science it was believed that patients had received infection from organisms that had developed in, and were received from a patient's environment. In other words, the invaders were saprophytes which incidentally possessed pathogenic powers. Such is apparently the actual case with some of the patho genic fungi. On the other hand, the pathogenic bacteria are apparently of a highly parasitic nature. Bacteriological researches have failed to demonstrate that they can live a sustained vegetative extracorporeal existence. When found without the body it is under circumstances which indicate they are existing within a definite radius surrounding an infected person, and their origin has been undeniably the excretions and secretions of the infected person, and it has been further found that these micro-organisms, with certain few notable exceptions, are only capable of a rather transitory extracorporeal existence, during which period their vital activities are more or less sus pended and their vitality tends to decrease.