by the use of the words "contagion" and "contagious" in describing those diseases which were considered to spread from one person to another by contact. "Infection" and "infectious" were limited to the cases in which the poison of the disease was supposed to be conveyed by the atmosphere from the sick person to those at a greater or less distance from him. Accordingly, we used to hear of a disease being contagious but not infectious, and vice versa. The distinction is, however, a purely artificial one, and is not sustained by facts, for many of the contagious diseases can be propagated indirectly—that is, without actual contact between the person who yields the poison and the person who receives it. Take, for example, diphtheria: it assuredly spreads by contact, and is therefore contagious, and no less positively is the poison capable of being disseminated through the atmosphere and infecting those who inhale it. So, too, with small-pox. If its virus be introduced under the skin of a person unprotected by vaccination or a previous attack, he will almost certainly suffer from the disease, and the same result would follow were such a person to be in close attendance upon a small-pox patient. In the latter case, the poison floating about in the atmosphere would get into the system through the lungs, and this is practically just as much an example of contact as if the poison were artificially introduced through the skin. It is therefore better to consider the terms "infection" and "contagion" as practically synonymous, and they will be so used in the remarks that follow.
To show what is implied by an infectious disease, let me take a typical example and contrast it with another disorder well known to be non-infectious. A young adult, previously in good health, is suddenly attacked by such symptoms as chilliness, soreness of throat, and evidences of derangement of the stomach. There is nothing characteristic about these symptoms; but let us suppose that on the following day there are high fever, dryness of skin, headache, giddiness, etc., and that in a few hours a scarlet rash appears, first on the chest, and then spreads over the body. All the symptoms become worse, and for ten or twelve days the patient is very ill. After this period, in favorable cases, a change takes place for the better, the rash dies away, and all the other symptoms gradually subside. In from four to six weeks, supposing that there are no complications, the patient regards himself as well. Such, in a few words, is the course of a mild case of scarlet fever, which may be considered as a typically infectious disease. Now suppose that our patient is treated in a house where there are several other young people who have never suffered from the disease. We know from experience that unless the most minute precautions are taken, the majority of these persons will exhibit similar symptoms. It is also well known that if any of these patients, supposed to have partially recovered from the disease, change their place of abode and go among other friends, the latter will run great risk of being attacked,
