4. The sources of infection as defined above may be classified as follows: (a) Typical Cases.—These are individuals in whom the dis ease presents the clinical manifestations typical of the disease.
(b) Atypical Cases (Abortive or Missed).—These are indi viduals in whom the clinical symptoms are indefinite or indistinc tive, and as a consequence are difficult to diagnose. Furthermore the illness is usually mild. Bacteriologic assistance is fre quently required for a diagnosis, though sometimes, when occurring as secondary cases where the primary case is known, they may be diagnosed clinically. Their proportion to the typical cases varies with different diseases.
(c) Carriers are apparently healthy persons who are harboring the specific organisms of communicable diseases and discharging them from their bodies. They are encountered in well defined types.
I. Incubatory carriers, i.e., individuals who are actually in the incubation period of a given infectious disease and will shortly manifest the characteristic symptoms.
2. Healthy carriers, i.e., individuals who throughout the entire period of their harborage never present clinical mani festations that may be referable to their parasitism.
3. Convalescent carriers, i.e., individuals who have recovered from an attack of the disease in question, but who continue to harbor the specific infective agents. According to the dura tion of their infectivity, convalescent carriers are commonly arbitrarily divided into two groups: (a) Temporary carriers, whose infectivity subsides in three months following the development of their convalescence, and (b) Chronic carriers, in whom the infective agents persist for periods longer than three months.
It must he remembered that the healthy and convalescent carriers possess an active resistance to the micro-organisms they are harboring, so that while they themselves are protected they endanger their associates.
The proportion of carriers to cases at any given time with a given disease is subject to wide fluctuations and is difficult of accurate estimation. The carrier condition, in so far as it re lates to the actual discharge of micro-organism is intermittent. The particular secretions or excretions are not continuously infective, even though the micro-organisms are continually present in the body. In this respect the carrier condition is closely related to the group of sources known as latent cases.
(d) Latent Cases.—With certain infections the active clinical manifestations tend to spontaneously subside, though the micro organisms themselves persist, apparently in a latent condition, for long and variable periods of time, later to renew their ac tivity when from some intercurrent cause their host's resistance is reduced. During the period of their latency they are not discharged from the body. Latent infection, for example, is
observed in syphilis, tuberculosis and glanders.
(e) Lower Animals as Sources of Infection.—The foregoing observations relate particularly to the so-called diseases of man, for which human beings serve solely as sources of dissemination. On the other hand, a certain group of human infections, such as rabies, glanders, anthrax, etc., are not due to agents derived from infected human beings, but from infected animals, though the role for the animal as a source of infection is closely parallel.
5. Examples of disease classification in the foregoing types: (a) Atypical forms are encountered in the following infections: Typhoid fever Poliomyelitis Septic sore throat Typhus Fever Measles Diphtheria Asiatic cholera Small pox Meningococcus meningitis Yellow fever Scarlet fever (b) Incubatory carriers have been observed in the following: Typhoid Fever Whooping cough Paratyphoid Fever Measles Malta Fever Small pox Dysentery (amoebic) Anthrax Diphtheria (c) Healthy carriers of micro-organisms producing the fol lowing diseases have been observed: Typhoid Fever Pneumonia Paratyphoid Fever Meningococcus meningitis Dysentery (amoebic and Gonorrhoea bacillary) Plague Cholera Poliomyelitis Diphtheria Malaria Scarlet Fever (d) Convalescent carriers of the following: Typhoid Fever Dysentery (bacillary and Paratyphoid Fever - amcebic) Diphtheria Cholera Pneumonia Meningococcus meningitis Poliomyelitis African Sleeping Sickness 6. Exit of from the Body of Infected Persons.
Different species of micro-organisms have adapted them selves to different pathways of exit from the body of infected persons. In order that the propagation of the species be main tained, it is of course necessary that colonizing individuals be disseminated. For the most part the adaptation is to the natural avenues of exit used for the physiological discharge of surplus secretions or the excretions of the body. To a consider able extent the adaptation is specific, i.e., certain of these path ways are more certain to contain micro-organisms producing a given disease, than others. For example the following dis charges are the means of exit of the infective agents producing: (a) Feces: Typhoid, dysentery, cholera, etc.
(b) Urine: Typhoid, etc.
(c) Sputum: Tuberculosis, pneumonia, etc.
(d) Saliva: Rabies.
(e) Nasopharyngeal secretions: Diphtheria, cerebrospinal meningitis, etc.
(f) Sweat: Typhoid.
(g) Milk: Typhoid, Malta fever, tuberculosis.
(h) Epithelial desquamations: Small pox.
(i) Purulent discharges: Scarlet Fever.
(j) Blood: Malaria, African sleeping sickness.
(k) Lachrymal secretion: Conjunctivitis.
(/) Placental circulation: Syphilis, tuberculosis.