questions. Congress has passed a comprehensive act, Section III of the Act of February 15, 1893, authorizing the Public Health and Marine Hospital Service to enforce interstate quarantines in the case of contagious and infectious diseases. The regulations, however, prepared under this act comprehend only the six quarantinable diseases, and have only occasionally been enforced in the case of yellow fever, cholera, or plague. There are no interstate regulations concerning typhoid fever, tuberculosis, measles, and other non-quarantinable diseases. It is evident that this is one of the important phases in which government activity can accomplish especial good; for, while the government has limited power within the state, it has practically unlimited authority so far as interstate relations are concerned. Widespread diseases will never be adequately controlled by the local authorities without the cooperation of the government. It is evident that, if one state should rid itself of typhoid fever, measles, or tuberculosis, it would soon become reinfected from the neighboring states. Interstate sanitation is one of the burning questions needing vigorous action and cannot be adequately enforced without extending the scope and powers of the present federal health authorities.

ISOLATION

In theory isolation is the most perfect single method to check the spread of a communicable disease. The results in practice, however, have been somewhat disappointing on account of unusual difficulties. The statement has frequently been made, especially with reference to typhoid fever, that if all the cases could be isolated (which includes the disinfection of the discharges) we would soon see an end of the infection. We now know that this statement is not true, on account of the bacillus carriers and the mild and unrecognized or "missed" cases. Because the isolation of the reported cases represents only a portion of all the foci of infection and, therefore, at best could not in itself control an epidemic disease, discredit has been thrown upon this procedure, which is one of the essential features of all systems of quarantine. As a matter of fact, it has been shown that in certain diseases, like measles, which is communicable for three days or more before the nature of the disease is recognized, isolation has practically no influence in diminishing the prevalence of this widespread infection. It is true ordinarily that a case of measles does most harm before it is isolated; nevertheless, this is no reason why it should be permitted to further endanger the community. The value of isolation is also dimin ished by the prevalence of carriers. In fact, its practical usefulness in a given infection is inversely proportional to the number of carriers.

If each case isolated prevents on the average only one other fresh infection, there would still be justification sufficient to continue the

practice. As a matter of fact, the practical value of isolation varies with each disease, depending upon the degree of its communicability, the time when it is communicable, the promptness by which it may be recognized, the modes by which it is transferred, the existence of latent infections, missed cases, carriers, and other factors which influence the spread of the infection.

The degree of isolation varies markedly with the different infections. A case of yellow fever may be isolated under a mosquito screen, and a case of diphtheria or scarlet fever may be effectively isolated in a bed in a general ward, provided intelligent and painstaking care is exercised to destroy the infection as it leaves the body. Isolation of the more readily communicable diseases, as smallpox and measles, should be much more absolute. Typhoid bacillus carriers need not be imprisoned. It is sufficient to limit their activities, especially to prevent their occupation in kitchens, dairies, or about foodstuffs. There is no good reason to isolate a consumptive or leper without open lesions-that is, cases in which the bacilli are imprisoned in the tissues and not discharged into the environment. A careful consumptive or leper may be allowed a wide latitude. On the other hand, isolation in chronic infections, such as tuberculosis and leprosy, with open lesions is the most helpful and at the same time the most difficult single procedure we have to control their spread. The careless, indigent, ignorant, or helpless consumptive is a public menace that needs energetic and sometimes arbitrary isolation.

Isolation may most readily and effectively be carried out in hospitals or sanatoria. Proper isolation in the home requires a special room or rooms, intelligent nursing, appliances for disinfection, etc., a combination often difficult to arrange. House quarantine varies with the different diseases. To carry it out rigorously in all cases and under all conditions is folly. Different diseases need different procedures. Sometimes it is sufficient simply to placard the house as a warning. At other times it may be necessary to station sanitary guards about the premises to enforce the quarantine. The imperfections of strict isolation by the "shutting in of houses" are graphically described in Defoe's "Journal of the Plague Year."

Isolation camps or temporary barracks in times of epidemics are effective measures in checking the spread of some infections. This method has proved effective in actual practice in the case of smallpox, yellow fever, plague, cholera, and other diseases.

It often becomes a difficult question to determine whether the well members of a household should also be quarantined-especially whether the well children should be permitted to attend school. This perplexing question must be decided for each disease separately, and the decision in each disease is sometimes modified by attending factors. Usually

the other children in the family in the case of scarlet fever are excluded from school for four weeks from the beginning of the last case. In most cities the same rule holds for diphtheria, although here we are able to determine whether the children are bacillus carriers or not. At least two negative cultures from the nose and throat should be required before such children are allowed freely to mingle with other children. The principal factors which determine whether the well children in a family shall be permitted to attend school or not in any particular infection rest upon our knowledge as to whether the disease is conveyed by a third person and the frequency of bacillus carrying and missed cases.

Isolation becomes one of our most valuable public health measures when communicable diseases affect persons working about milk, meat, and other foods capable of conveying infection.

One of the practical objections to isolation and one reason that it meets with so much opposition from the public is that the compensation of the wage earner ceases through no fault of his own. It is evidently unjust to practically imprison and thus seriously punish a member of the community, not for his own good but for the good of the community, because he or some member of his family has contracted an infection, perhaps through some fault of the community itself. It is, therefore, reasonable and just that wage earners and others should be compensated and their personal interests safeguarded during enforced isolation.

Isolation only reduces to a moderate degree the prevalence of discase. The limitations of this valuable procedure are now well understood. With improved methods of diagnosis and increased knowledge of the methods of spread of disease, isolation will be made increasingly effective. Every case isolated is a focus of infection neutralized. Although not as satisfactory in practice as it is in theory, isolation will ever remain one of the chief administrative procedures for the control of the communicable diseases.

SECTION II

IMMUNITY, HEREDITY, AND EUGENICS

CHAPTER I

IMMUNITY

Immunity or resistance to disease is the very foundation of preventive medicine. It is the overshadowing factor in hygiene. In this sense we use the term "hygiene" to include the care of the person, in contradistinction to "sanitation," which deals with the environment. There is no sharp line of demarcation-we speak of hygiene of the teeth, of sleep, of bathing, of exercise, or food and drink, and of those conditions which are more or less intimately associated with the body; we speak of the sanitation of the home, of schools, of cities, of farms; sanitary science considers the air, soil, climate, and our surroundings as they affect health. Sanitation, then, is largely impersonal; hygiene is personal, and, as far as the prevention of disease is concerned, one of the most important factors in hygiene is immunity.

The word "immunity" is a very old term-we still speak of immunity to crime, but it is only of late years that we are beginning to understand the mechanism by which the body protects itself against infection. The advances have been so rapid that these studies may now be grouped into a separate science known as Immunology.

Immunity is a function of all living beings (animals and plants), and in its widest form is one of the fundamental properties of life. Thus, as long as we are alive the colon bacillus in our intestinal tract and the spores of the hay bacillus on our skins do us no harm, but the moment we die, and ofttimes shortly before death, these and other bacteria invade our tissues and disintegrate them.

2

Immunity may be defined as the power which certain living organisms possess of resisting infections. Immunity is the contrary condition to susceptibility. Hypersusceptibility is a special state of an exag

1

We may speak of immunity "from" a disease, "to" a disease, and "against" a disease.

Terminal infections.

gerated power of reaction and will be discussed separately under anaphy laxis or allergie. The word resistance has practically the same signification as immunity. The term "tolerance" is commonly used to describe a limited form of immunity usually acquired by the repeated use of alkaloids, alcohol, and other poisons of comparatively simple chemical structure. While a high degree of tolerance may be acquired to such substances, a true immunity in the sense in which the term is now used is never produced. In the case of tolerance, antibodies are not found in the blood. For the most part true immunity is produced against colloidal substances, while tolerance is largely limited to the crystalloids; this distinction, however, is not absolute.

There are all gradations and various kinds of immunity. It varies in degree from the weakest appreciable amount to an almost absolute protection. It also varies greatly in duration-from the briefest period to a life span. Immunity, therefore, is a relative term. It may be natural or acquired, active or passive, local or general, pure or mixed, specific or general, family or racial, brief or lasting, strong or weak, etc.

Immunity is a function which is not limited to man and other members of the animal kingdom. It is common throughout the vegetable kingdom. We are indebted to Welch for the thought that the bacteria themselves also have this fundamental property of life. Thus, man is susceptible to the tubercle bacillus because the tubercle bacillus is immune to man; on the other hand, man is immune to the hay bacillus because the hay bacillus is susceptible to man. In this sense a microorganism is called pathogenic or non-pathogenic, depending upon whether it harms or is favored by its host. This is the relation between seed and soil. A fertile soil is susceptible; a barren soil is immune. The seed in the first instance may be pathogenic; in the second non-pathogenic. The host is able to resist the intrusion and growth of the non-pathogenic microorganisms and protect itself against harm through its mechanism of immunity. If the protecting devices are insufficient to guard against attack, the germs multiply, produce poisonous substances, or harm the host in other ways. The reason that the same microorganism may be pathogenic for one host and harmless for another depends upon the presence or lack of immunity. The virulence of a microorganism is an expression of the intensity of the relation between the seed and the soil. Virulence may be strengthened or attenuated either by increasing or decreasing the resistance of the host or by increasing or decreasing the resistance of the microbe.

Mechanism of Immunity-Theories of Immunity.-It is now quite evident that the mechanism of immunity varies in different infections and, to a certain extent, even in the same infection under different conditions. It must further be admitted that we are still in ignorance of