THE DISSEMINATION OF INFECTIVE AGENTS I. Stage of Disease in Relation to Dissemination.—(a) Stage of Invasion and Incipiency.—With some diseases such as tuberculosis, when the micro-organisms are in deep seated lesions not in contact with passage ways to the exterior, the infected person is of little or no active danger to others as a source of infection. On the other hand, with diseases such as diphtheria, where the micro-organisms are in superficial situations having ready access to the exterior, this period is of great importance.
(b) Non-infective Stages of Active Disease.—When syphilitic infection progresses to the tertiary and para-syphilitic stages, these as long as they persist are non-infectious. On the other hand, they partake of the character of latent infections, inas much as relapses to lesions of the secondary type may occur.
2. Portals Through Which Infective Agents Enter the Body.— Different infective agents have adapted themselves to varying portals of entrance into the body and for the most part, if introduced by a route other than the one to which adapted, fail to gain a foothold. The principal body orifices again play an important part, particularly the orifices of entrance, rather than those of exit. After having passed the threshold of this portal, the infective agents may proceed by divergent pathways to penetrate the physiological interior of the body.
The mouth and the nose are portals of entrance of greatest importance from the standpoint of the number of infective agents which are introduced through them.
After passing this threshold, some, as the typhoid bacillus may go onward to the intestinal tract, others as the pneumo coccus procede to the respiratory passages, while others as the diphtheria bacillus, will remain near the point of their entry in the naso-pharynx.
Areas of specialized epithelium, as for example that of the conjunctiva or genitalia, are particularly well suited to some infective agents.
Others, by various means, usually due to some form of traumatism, (inoculation) directly penetrate the outer epithe lium of the body to the subcutaneous tissues, and thence gain access to the circulation or to the central nervous system.
3. mere introduction of a given infective agent into the body will not necessarily mean that the micro organisms will succeed in gaining a foot-hold. The success of a given transfer will vary directly with the number and virulence of the organisms transferred, and indirectly with the resistance of the individual invaded. These factors also in fluence the duration of the period of incubation.
4. The Extracorporeal Existence of Infective infective agents are for the most part highly specialized para sites that require for their existence the conditions of tempera ture, humidity, moisture, and nutrition that they encounter within the body of their host. When these conditions are not
available their vegetative and reproductive activities become sluggish, and if prolonged the vitality of the organism may become impaired. Furthermore the colonizing individuals when expelled from the body of their host are ordinarily imme diately exposed to the action of the natural germicidal agencies, sunlight and desiccation. (See Table II.) In consequence of these factors they become rapidly attenuated and are soon destroyed. It is exceedingly rare for them to gain introduction into any medium favorable for multiplication, such as milk. The fortunate individuals who do succeed in reaching a new host are but a small fraction of those which left the source. These circumstances are our salvation.
It must of course be remembered that the spore-forming bacteria such as the anthrax and tetanus bacilli, are exceptions to the rule in so far as survival is concerned, but on the other hand, their numbers probably do not increase during this period.
Exception must also be made to certain animal infective agents. These which have a definitive host of course multiply in that species. A few of the multicellular parasites require an extracorporeal period to complete certain stages of their life cycle and are particularly adapted to this. Their degree of specialization as parasites is not as extreme as it is with the parasitic bacteria.
Therefore as a general rule, the closer the relationship in time and space with the source of infection, the greater is the chance or the successful transfer of unattenuated infective agents.
5. Routes of Transference.—The successful transfer of in fective agents from host to host is a lottery in which all the chances are against the individual micro-organisms. The perpetuation of the species is secured by the discharge of tre mendously enormous numbers of individuals from the infected source.
Their transfer is effected by means which serve unconsciously to carry human secretions from one person to another. These means may be classified into the following groups in order of their importance. They are: (a) Contact (b) Foods, including Water and Ice.
Milk and Dairy Products.
Meat and Shell fish.
Other Foods.
(c) Insects Mechanical transmission.
Biological transmission.
(d) Soil.
(e) Fomites.
By the later term we understand objects upon which infective agents may retain their vitality for prolonged periods of time. As a matter of fact, this means of transmission is closely related to certain forms of contact and is only of importance with the spore-forming infective agents.