The first chapter of this book described the ways in which communicable diseases are carried from person to person, and also some principles underlying methods of prevention. This chapter aims to show how these principles apply in the actual care of patients whose diseases are transmissible. In order to apply them intelligently, it is necessary to keep in mind certain facts in regard to the transmission of infections. A brief summary of these facts follows. Disease germs are present in the bodies of persons suffering from communicable disease, but they may also exist in the bodies of persons in good health; if present in the body, they may leave it in any bodily discharge. While every kind of germ does not leave the body by all the different routes, it is nevertheless true that most germs expelled from the body are carried in discharges from the nose, throat, bladder or bowels. Germ-laden discharges of an infected person may be distributed to other persons by water, milk and other foods, by certain insects, by unclean hands, by common drinking cups, towels, handkerchiefs, and similar articles, and directly by nose and throat spray. After they have been thus conveyed to other persons, the germs make their entrance into the body of their new victims through the digestive tract, through the nose, throat, and other mucous membranes, or through breaks in the skin. Prevention of communicable diseases, therefore, depends upon the measure of success attained in blocking the transit of germs from person to person; but methods of prevention, though easy to understand, are unfortunately sometimes difficult to carry out. In order to carry them out effectively one must devote to the problem great accuracy, unremitting care, considerable intelligence, and a highly developed conscience. Care of a patient suffering from transmissible disease is adequate only when it accomplishes two definite results. One result, which concerns the patient primarily, is to bring about his recovery as rapidly and as surely as possible; the other result, which concerns the community rather than the individual, is to make it impossible for the patient to infect others with his disease. In every case of communicable disease, from a slight cold in the head up to serious cases of pneumonia or typhoid fever, both the patient and the community must be constantly safe-guarded. INCUBATION PERIOD.--The interval between the moment when pathogenic germs enter the body, and the time when symptoms first appear and the patient begins to feel ill, is called the incubation period. Incubation periods vary according to the disease from a few hours to two or three weeks. The length of the period also varies somewhat in different cases of the same disease.