On the other hand the unrecognized infected persons are commonly those who experience little or no inconvenience as a result of the infection. As a consequence the radius of their activities is reduced but little, they rarely consult a physician, their infectivity is seldom recognized and hence no precau tionary measures are employed. Their infective secretions are as a consequence scattered over a wide radius. Thus they are of major importance in the propagation of infective agents.
It is therefore apparent the measures of isolation directed against the sources of infection will be successful in inverse proportion to the number of unrecognized infected persons at large in a community.
6. Features Characteristic of Contact The greater the intimacy of association between human beings, the better and more frequent will be the opportunities for the transfer of infective agents by contact. The incidence of con tact disease is commonly in direct proportion to the density of population in a given area.
(b) The intimate association of people at home, at school or in different institutions favors the spread of infection by con tact. This aspect is closely associated with housing problems, with over-crowding and inadequate lighting and ventilation, both of which tend to prolong the vitality of pathogenic micro-organ isms outside the body. Fig. r illustrates housing conditions intensified by overcrowding, that favor the dissemination of infective agents by contact. Fig. 2 illustrates the definite rela tion of overcrowding to disease incidence, as influenced through contact transmission in the case of measles. Under these cir cumstances it will frequently be possible to observe a primary and secondary relationship between the cases that arise, the primary case of course serving as the local source of infection.
(c) It is to be noted that most of the so-called "children's diseases are included in the group, "commonly transmitted by contact." In other words, these diseases are most commonly
observed during the period of childhood. Several explanations for this fact may be advanced. First, it may be due to the well nigh universal susceptibility of children as compared with the active acquired immunity possessed by most adults. Or, since it is well recognized that the promiscuous habits of carefree childhood permit greater opportunities in the unrestricted trans fer of secretions, it is not improbable that the greater opportuni ties for contact transfer during this period are of considerable importance in this age incidence.
(d) Contact outbreaks or epidemics are never explosive in character, they are always relatively slow spreading, with a gradual rise and decline. This is due to the multitudinous channels through which the infective agents are disseminated, few individuals being simultaneously infected from one source as contrasted with the explosive character of outbreaks follow ing the distribution of infected water or food, where a single contaminated vehicle has simultaneously infected a considerable number of persons.
(e) Seasonally, contact diseases tend to reach a maximum prevalence during the colder period of the year. At this period the inclement weather forces the human race within doors, where the degree of personal association is more intimate than that during the warmer period when a greater portion of the time can be spent out of doors. This seasonal distribution is also closely associated with the operation of the public schools. It is also to be noted that continued periods of inclement weather, particularly that characterized by excessive cloudiness is often associated with an increase in the amount of contact transmission, probably by forcing people within doors, as well as by permitting the longer survival of infective agents outside the body, due to the diminished sunlight.