7 66 THE POPULAR SCIENCE MONTHLY. 



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 con- 

 veyed 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 assured- 

 ly 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 vacci- 

 nation 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 poi- 

 son 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 sud- 

 denly attacked by such symptoms as chilliness, soreness of throat, and 

 evidences of derangement of the stomach. There is nothing charac- 

 teristic 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, 



