Vaccination and inoculation have saved thousands of lives. Smallpox, once more prevalent than measles, was the scourge of Europe until vaccination was introduced. During the 18th century it was estimated that 60,000,000 people died of it, and at the beginning of the 19th century one-fifth of all children born died of smallpox before they were 10 years old. In countries where vaccination is not practised the disease is as serious as ever; in Russia during the five years from 1893-97, 275,502 persons died of smallpox, while in Germany where vaccination is compulsory, only 8 people died of it during the year 1897. Death rates from diphtheria and typhoid fever have been greatly reduced by the use of antitoxin and antityphoid vaccine. Thus in New York State in 1894, before antitoxin was generally used, 99 out of every 100,000 of the population died of
diphtheria, while only 20 out of 100,000 died of it in 1914. In 1911 a United States Army Division of more than 12,000 men camped at San Antonio, Texas, for four months. All of these men were vaccinated against typhoid fever and only a single case occurred during the summer, although conditions of camp life always tend to spread the disease.
While many and various factors tend to lower resistance rather than to increase it, the idea that these factors act equally in all kinds of infection is erroneous.
"The principal causes which diminish resistance to infection are: wet and cold, fatigue, insufficient or unsuitable food, vitiated atmosphere, insufficient sleep and rest, worry, and excesses of all kinds. The mechanism by which these varying conditions lower our immunity must receive our attention, for they are of the greatest importance in preventive medicine. It is a matter of common observation that exposure to wet and cold or sudden changes of temperature, overwork, worry, stale air, poor food, etc., make us more liable to contract certain diseases. The tuberculosis propaganda that has been spread broadcast with such energy and good effect has taught the value of fresh air and sunshine, good food, and rest in increasing our resistance to this infection.
"There is, however, a wrong impression abroad that because a lowering of the general vitality favors certain diseases, such as tuberculosis, common colds, pneumonia, septic and other infections, it plays a similar rôle in all communicable diseases. Many infections, such as smallpox, measles, yellow fever, tetanus, whooping-cough, typhoid fever, cholera, plague, scarlet fever, and other diseases, have no particular relation whatever to bodily vigor. These diseases often strike down the young and vigorous in the prime of life. The most robust will succumb quickly to tuberculosis if he receives a sufficient dose of the virulent micro-organisms. A good physical condition does not always temper the virulence of the disease; on the contrary, many infections run a particularly severe course in strong and healthy subjects, and, contrariwise, may be mild and benign in the feeble. Physical weakness, therefore, is not necessarily synonymous with increased susceptibility to all infections, although true for some of them. In other words, 'general debility' lowers resistance in a specific, rather than in a general, sense."—(Rosenau: Preventive Medicine and Hygiene, pp. 403 and 404.)
CARRIERS
Well persons who carry in their bodies pathogenic germs but who themselves have no symptoms of disease are called carriers. Thus typhoid carriers have typhoid bacilli in the intestinal tract, while they themselves show no symptoms of typhoid fever; diphtheria carriers have bacilli of diphtheria in the throat or nose, but have themselves no symptoms of diphtheria, and so on. It has now been proved that many patients harbor bacteria for weeks, months, or even years following an infection, and are dangerous distributors of
disease; also, some healthy individuals without a history of illness harbor living bacteria which may infect susceptible persons in the usual ways. Transmission by healthy carriers goes far to explain the occurrence of diseases among persons who have apparently not been exposed. This explanation has greatly clarified the whole problem of the spread of communicable diseases. Carriers, unfortunately, exist in large numbers, and render the ultimate control of disease exceedingly difficult. They can usually be identified by bacteriological tests. To some extent they can be supervised; food handlers at least should be legally obliged to submit to physical examinations, and should be licensed only when proved free from communicable disease.
Diseases are also spread by persons suffering from them in a form so mild or so unusual that they pass unrecognized. These persons are known as "missed" cases. Carriers of disease and "missed" cases go freely about the community, handling food, using common drinking cups, travelling in crowded street cars, standing in crowded shops; in various ways coming into close contact with other people, coughing and sneezing and kissing their friends no less often than normal individuals. It is consequently clear that the bodily discharges of supposedly normal persons
may be hardly less a menace than those of persons known to be infected.
Diseases that depend for transmission upon milk, water, food, and insects may be controlled by public action, that is, by specific measures taken by a large group of people in order to protect the individual. Such action constitutes public sanitation. There is, however, a large group of diseases, chiefly sputum-borne, that cannot be controlled except by individual action. Such individual action constitutes a large part of personal hygiene.