(a) Diseases Transmitted Solely by Direct Contact.—This group includes the venereal diseases, syphilis and gonorrhoea, which from a practical standpoint are propagated nearly ex clusively by sexual intercourse. Under these circumstances fresh highly infective secretions are transferred.
(b) Diseases Usually Transmitted by Contact.—This group includes the common so-called " contagious " diseases, for example, small pox, chicken pox, mumps, whooping cough, measles, diphtheria, scarlet fever, common colds, influenza, tuberculosis, meningitis, pneumonia, poliomyelitis, etc. It is known that several of these may be transmitted by other routes of infection, but with all contact transmission is the principle means by which the infective agents are distributed.
(c) Diseases Frequently Transmitted by Contact.—With these contact transmission is quite common and is of far greater importance in the propagation of their infective agents than is generally recognized. The spectacular, explosive outbreaks due to the occasional activity of other routes of infection have overshadowed the quiet unpretentious activity of contact in the continued propagation of their infective agents. To this group belong typhoid fever, Asiatic cholera and dysentery (Fig. 3).
4. It is to be noted in the foregoing classification that the varying importance of contact transmission is clearly associated with the route of exit from the body to which different species of infective agents are adapted. As we shall endeavor to show, this naturally influences the distribution of the infective agents.
In the first of the foregoing groups, namely the venereal diseases, the active lesions are ordinarily present on concealed portions of the body, on areas covered by the clothing which tend to prevent the very general distribution of infective secre tions. Transfer of the infective secretions comes as the result of an act, usually voluntary, which places two individuals in such relationship that transfer of the infective secretions is rendered not only possible but probable.
In the second of the above groups it will be later learned that the most unusual avenue of exit selected by these micro organisms, at least the predominating avenue, is that offered by the secretions and excretions leaving by the mouth and nose.
The physiological structure of this region is such that these secretions do not accumulate until of considerable volume, but rather the accumulations are small and are expelled at frequent intervals. Furthermore these orifices are not covered by arti ficial devices, so there is no impediment to the frequent expul sion of these secretions, neither do any motives of modesty or artificial impediments prevent the introduction of the fingers into either the mouth or the nose. As a consequence these secretions are distributed thinly over a radius corresponding to the daily movements of their producer.
In the third group the infective agents chiefly leave the body in either the feces or the urine, or both. These excretions are not expelled continuously but accumulate until the capacity of either the rectum or bladder is reached; thereupon they are expelled. Furthermore, from considerations of modesty these orifices are protected by the clothing, which reduces the fre quency of finger contamination with these excretions, as well as the frequency with which these discharges are voided. As a consequence these excretions are distributed thickly but irregu larly over a relatively small radius.
5. Importance of the Different Types of Sources of Infection in Contact Transmission.—The known cases are usually of minor importance in the propagation of these diseases for several reasons. As a result of the invasion they feel ill, consult a physician who usually recognizes the infective character of their illness and directs that a more or less effective regimen of isolation be observed. Even in the absence of a physician, the severity of their illness will tend to restrict their movements or the radius of their activities, and hence temporarily reduce the number of their associates. Both of these factors materially reduce the importance of these patients in the further propaga tion of their infection, unless perchance they become convales cent carriers.