(d) One of the most useful means of promptly checking the unusual occurrence of disease, that is of recognizing incipient epidemics in order that effective control measures may imme diately be inaugurated, is the so-called endemic index. The endemic index is the average number of cases occuring in a given area during a given period of time, in each of five con secutive years exclusive of epidemics. The average secured is used for comparative purposes with the number of cases re ported in later periods.
6. Reciprocal Notification.—Present day means of trans portation are steadily progressing in speed. As a consequence a person infected on the other side of the world may travel half the globe before the expiration of the period of incubation. Consequently infective agents can be transported great dis tances by apparently healthy persons. Furthermore those who are actively infected, either cases or carriers, recognized or unrecognized, may travel about evading the health authorities and spread infective secretions in their path. Health officers can secure valuable assistance in detecting the possible spread of infective agents by travellers from distant points, by keeping informed of the prevalence of these diseases in jurisdictions other than their ONV11. In the United States a knowledge of the prevalence of reportable diseases in other jurisdictions may be gained from the Public Health Reports published weekly by the Federal Public Health Service. This bulletin also publishes world wide summaries of the occurrence of plague, cholera, typhus, and small-pox, which latter summaries are also published in the Monthly Bulletin of the International Office of Public Health Hygiene, published in French from Paris. In addition several of the more progressive State Boards of Health notify other health jurisdictions of cases of reportable diseases developing within their bounds in recent arrivals from these outside jurisdictions. By this means sources of infection frequently unknown to these authorities are revealed.
7. Investigation of Cases Reported or Discovered.—It should be the purpose of every health officer to endeavor to discover the source of infection responsible for every case of disease recognized. This can only be accomplished by an
epidemiological investigation of every case reported imme diately upon receipt of the report, simultaneously with the inauguration of appropriate control measures. The inves tigation should have the following scope in order to reveal the desired information, and of course its scope will vary depending upon the routes of transmission possible with different infective agents. The information sought should include: (a) Clinical Data.--Previous attacks of the same disease, date first felt ill, date took to bed, date diagnosis achieved, date of recovery or death, and other special information illu minating these points.
(b) Possible Sources of Infection or Routes of Injection En countered.—Contact: Exposure to known or suspected cases, other similar cases at school or place of employment, similar cases in the same neighborhood etc., or in the same family, household, or among other associates; travel; social activities etc.
Water, milk and food consumed. Usual and unusual sources of supply, at home, place of employment, school, or travel etc. Where unusual sources of supply are ascertained, particular endeavor must be made to localize the date of their consumption.
Flies and other insects in relation to a possible source of in fection as well as from the standpoint of further dissemination of infection from the patient.
(c) Opportunities for Further Spread of Infective Agents from the Patient.—Susceptibles in contact with patient from the probable beginning of his infectivity; the dates upon which isolation and concurrent disinfection were begun; the dates upon which they ceased; and the hygiene and sanitation of the patient's dwelling.
A form used for the investigation of typhoid fever is shown on pages 87-9o.
Upon the scope and detail of the interrogations will largely depend the success of the investigation in revealing the probable source and route of infection. When all information is gathered from the several cases of an epidemic it must be subjected to a statistical analysis to secure information relative to the route of infection involved and the probable source of infection. The investigation will also result in the discovery of missed cases.