The change has been brought about by the discovery that the germs of yellow fever are carried by a certain species of mosquito which transmits the infection from a diseased person to others. The cleaning-up of stagnant pools in which the mosquitoes breed, and the careful screening of doors and windows has practically eliminated the disease wherever these measures have been taken. Substantially the same thing is true of the disease known as malaria so far as causes and remedies are concerned. Typhus, a fever which has long been the pestilence of backward countries, is transmitted from person to person by the common body-louse or “cootie”. Cities, armies, and even whole countries have been set free from this plague by delousing operations, that is by the wholesale disinfection of clothing and persons.[[254]] Bubonic plague, the Black Death of the Middle Ages, which has swept over Asia and portions of Europe so many times, is transmitted by rat fleas. Other diseases besides yellow fever, typhus, and bubonic are known to be spread by insects, and still others are believed to be. The common house-fly is undoubtedly a carrier of typhoid germs from filth and sewage to the water, milk, and food supplies in homes and stores. The world would be far better off if the whole category of disease-carrying insects, mosquitoes, lice, fleas, and flies, could be made as extinct as the dodo.

The Wide Range of Disease and Causes.—Not all diseases, of course, are caused by polluted food and drink or by the transmission of insect-borne bacilli. Some are undoubtedly caused by pathogenic germs whose methods of infection we do not yet know. There are various theories as to what carried influenza from one end of the world to the other during 1918 but none of them satisfactorily explain all that happened. The epidemic in one case broke out upon a sailing ship, far off at sea, six weeks after it had left port. So disease still has its mysteries, yet unsolved. |Things which cause and spread disease.| We know, however, that many ailments are either directly caused or are facilitated by poor nourishment, bad ventilation, lack of cleanliness, physical exhaustion, and lax attention to personal hygiene. Some are due also, in whole or in part, to certain forms or conditions of work, and are commonly known as occupational or industrial diseases. Such, for example, is lead poisoning in paint factories; such also is the illness which often overcomes men working in tunnels and other places where compressed air is used. The spread of tuberculosis, the great white plague of today, is undoubtedly due in some measure to dust and bad ventilation. The relations between diseases and occupations have not received careful study until recent years, but it is believed that we shall ultimately find the causes of many human ailments in the conditions under which some forms of industry are carried on.

Industry and ill health.

Hygiene of Factories and Workshops.—Because of the unsanitary conditions which have been found by investigation to exist in workshops and factories, particularly in the large cities, various states have made laws and regulations to protect the health of employees in such establishments. Some trades, such as the making of poisonous phosphorous matches, have been prohibited altogether. Others, by reason of their danger to the health of the workers, have been subjected to strict regulation. The “sweat-shops” or tenement rooms in which women and children formerly worked long hours for a mere pittance, crowded together with almost no ventilation—these industrial dungeons have been legislated out of existence almost everywhere. Workshops and factories must now be commodious, well-lighted, clean, and properly ventilated. Adequate sanitary equipment must be provided. It is the duty of the state factory inspectors to see that all these requirements are fulfilled.

The Prevention of Disease—Individual Precautions.—Without the co-operation of individuals no government can maintain a high standard of health among the people. All that the public authorities may do will prove inadequate unless individuals themselves, young and old, understand and observe the means of disease-prevention. |Physical fitness.| Physical fitness is one of the greatest blessings any man or woman can have, and it is largely the product of strict attention to the upbuilding of the body in early years. Theodore Roosevelt was a frail, sickly lad in his boyhood days. He realized, as he explains in his autobiography, that he could never make a marked success in life without building up his physical vigor, so he set about doing it, and by the time he had reached full manhood he was a model of physical ruggedness. How did he manage it? Regular habits, out-of-door life, prompt attention to minor ailments, a zest for every form of wholesome sport,—these things transformed a weakly youth into the sturdiest man that ever sat in the presidential chair.

There is no need to lay down any definite rules as to how the young men and women of America may gain and maintain a high standard of health and bodily vigor. Common sense will suggest most of them. The besetting sin of youth is its prodigality, the wasting of strength that should be saved for years to come, and the failure to realize that an individual’s health at the age of forty or fifty depends very largely upon what use is made of health opportunities during the years from fifteen to twenty-five. The glory of a young man is his strength; he does not usually let his mind run ahead to the day when he will be neither young nor strong. No investment that young people can make will pay higher dividends than that which is represented by the time, the thought, and the care spent upon the task of keeping well in early years.

Quarantine.

The Prevention of Communicable Diseases: Quarantine and Disinfection.—First among the measures taken by the public authorities to prevent the spread of communicable diseases are the quarantine regulations which are enforced at all the seaports under the authority of the national government. Day and night throughout the year, the health officers stand guard at these ports to see that no disease-bearing persons are permitted to land. Vessels leaving foreign harbors for the United States must secure a bill-of-health from the American consul before they sail; and the first person who goes on board an incoming vessel after it takes on its pilot is the quarantine officer. This official permits no passengers to be landed until he has made sure that there are no persons afflicted with communicable disease aboard. If there are any such cases, the passengers are held until the danger is past. The various states and cities also maintain systems of health-inspection and quarantine. Certain diseases (including tuberculosis, smallpox, typhoid, scarlet fever, pneumonia, whooping cough, diphtheria, measles, mumps, and so forth) must be promptly reported to the local health authorities. The local health regulations require that in case of the more readily communicable diseases the house be placarded. In extreme cases, the patients may be removed to an isolation hospital.

Disinfection.

After the illness has terminated, the regulations usually provide that the premises shall be disinfected under the supervision of an official from the health department. Every state, as well as every city and town, maintains general regulations relating to quarantine and disinfection, these being enforced by the state health authorities. For the most part, however, this work is supervisory, the detailed enforcement of the rules being left to the health officers of the various communities, although in the case of epidemics, involving several municipalities, the state health authorities usually assume direct control. Similarly, when epidemics spread or threaten to spread from one state to another, the national health authorities step in.[[255]]