8. Analysis of Epidemiologic Data.—In undertaking such an analysis it is important to locate the cases both from the stand point of place and time, as this will yield important information, First however, the imported cases should be excluded from the analysis. A case is regarded as if imported if either (a) brought into the local area while ill, or (b) if out of town at at the probable time infection was received. The principal consideration they should receive will relate to their potentia lities as a local source of infection.
To localize the cases from point of space they should be plotted upon a map of the district according to their place of illness at the time of onset. One should then note if their dis tribution is localized or generalized, and if it has any definite relation to possible neighborhood activities, or water or milk supplies. From the standpoint of time the case should be plotted upon co-ordinate paper according to their dates of onset. From the curve thus secured one should note whether the course of the epidemic is abrupt or explosive in its rise and fall, or slow and gradual. The former curve indicates that the cases owe their infection to the use of some common route of infec tion, such as water, milk, etc., in other words that their infec tion has been simultaneous. The slow rise and fall of the curve indicates contact transmission or fly transmission.
The contact transmitted diseases with short incubation periods and great virulence, such as influenza, will produce an epidemic curve much more abrupt than that produced by the other diseases of this group, having longer incubation periods.
If the curve is explosive, analysis of the data secured under the heading of probable routes of infection, should be made to ascertain the route of infection common to all cases, at least those of the explosive period. When ascertained this must be further investigated to reveal the source from which the route became contaminated.
On the other hand, if the outbreak had a slow rise and fall the contact histories will for the most part link up with one another, excepting for gaps produced by transfers of infective agents from unrecognized infective persons. Epidemics due to different types of routes of infection have certain peculiarities, to which attention will be called when considering these routes in detail.
As soon as the infected route of transmission is ascertained proper measures to render it innocuous must be immediately inaugurated. When this is done the epidemic will draw to a
close except perhaps for a few scattered contact cases.
The probable time of infection is reckoned by subtracting the average incubation period from the date of onset.
9. Emergency and Routine epi demiology is the application of the principles presented in the foregoing paragraphs and chapters in the face of the emergency produced by an epidemic, and epidemics usually result from a deficient or inadequate public health organization. The work is usually done in the face of the epidemic, actually at a time when the danger is at a maximum or tending spontaneously to decline. The remedial measures employed are usually of a temporary character, aiming principally at a control of routes of infection rather than sources.
On the other hand, the routine application of the foregoing methods day after day to each case of communicable disease recognized, will soon show results in a reduction of both morbidity and mortality, and extensive epidemics will be prevented. Small epidemics may occur due to unrecognized importations of infection, but these will not have a chance to spread. This is routine epidemiology, which not only aims at a permanent control of the routes of infection, but of the sources as well.
to. Education of the Public in General education of people in personal hygiene will do much to pre vent the spread of diseases, as well as secure their co-operation with health officials. The following points in general cleanliness are of special importance in this connection: (a) Washing the body frequently with soap and water.
(b) Washing the hands with soap and water after voiding the bowels and bladder, and always before eating.
(c) Keeping the hands and unclean articles, or articles that have been used for toilet purposes by others, away from the mouth, nose, eyes, ears and vagina.
(d) Avoiding direct exposure to the spray from the noses and mouths of people who cough and sneeze, or laugh and talk loudly or in an explosive manner.
(e) Avoiding the use of common or unclean eating, drinking, or toilet articles of any kind, such as towels, hair brushes, drink ing cups, pipes etc.
(f) In epidemic periods public assemblages should be dis couraged and the wearing of masks should be encouraged.
(g) Promiscuous spitting should be prohibited.