and that the disease may thus spread ad infinitum. This capacity of propagation, the possession of which is as certain as anything can possibly be, suggests the inquiry as to the manner in which the original patient of our series became infected. He in his turn must have taken the disease from some one else, but it is quite possible that he has never been within a mile of a scarlet-fever patient. In many such instances it is impossible to get any clew to the original case, but it sometimes happens that evidence is forthcoming to the effect that days or weeks, or even months before, a person convalescent from the disease has occupied a room of which our patient was afterward a tenant, or that some article of clothing which once belonged to patient number one has been handled or worn by the person whose case we are considering. It is evident that there must often be great difficulties in prosecuting such an inquiry.
Let us now take an example of a non infectious disease, and notice how it contrasts with the one we have just described. A young adult, previously in good health, becomes sensible of a feeling of heat, alternating with chilliness, and perhaps shivering, and slight pains in the limbs. In a day or two there is more or less fever and thirst, and some of the larger joints are swollen and very painful, while the skin covering them is much reddened. The pain and fever are the principal symptoms; but there are often others, a description of which is unnecessary for our present purpose. The complaint lasts an indefinite time, but, even in the absence of treatment, usually subsides within six weeks. Such, in a very few words, is the course of rheumatic fever or acute rheumatism.
These two diseases, scarlet fever and rheumatic fever, have much in common, but there are sharp points of difference between them. In both fever is a prominent symptom, and, in addition to the display of local symptoms, the whole system is evidently affected. The differences, however, are still more important. Scarlet fever is eminently infectious. The air which surrounds the patient becomes contaminated and highly charged with the poison, and persons breathing it run great risk of becoming affected. In a case of rheumatic fever, although the secretion from the skin is generally very copious and peculiar in character, so that the sense of smell is strongly appealed to, there is no such risk; the disease can not be conveyed from the patient to those around him, however close the attendance and however defective the ventilation of the room. Infection from a previous case is, therefore, never thought of in connection with rheumatic fever, though the actual nature of the poison which causes the disease is as yet unknown. The attack is often excited by exposure to cold and wet, circumstances which play no part in the causation of scarlet fever. There is at least one more important difference between the two diseases: scarlet fever very rarely, indeed, occurs a second time in the same patient, and the symptoms never become chronic; rheumatic fever, on
