1 6 TEE POPULAR SCIENCE MONTHLY 



shown to be quite practicable, in temperate climates at least, by the 

 drainage of marsh lands and the filling or oiling or stocking with fish 

 of the smaller mosquito breeding pools. 



The third mode of infection, by contact, the more or less direct 

 transfer from person to person, is by far the most important factor in 

 the spread of communicable disease in temperate climates. Malaria 

 is our only important insect-borne disease. Typhoid may sometimes be 

 spread by flies and often by water or milk — diphtheria and scarlet 

 fever and tuberculosis and tonsilitis, sometimes by milk. On the other 

 hand, diphtheria and scarlet fever and tuberculosis and, in cities with 

 good water supplies, even typhoid fever, are all more commonly trans- 

 mitted by contact than in any other way ; and contact is practically the 

 sole cause of smallpox, measles, epidemic cerebro-spinal meningitis, 

 influenza, common colds and the venereal diseases. It is in the nose and 

 throat, or on the hands of a human being, that disease germs generally 

 enter our homes, and this sort of infection is obviously most difficult to 

 detect and control. The child infected with measles yet showing no 

 symptoms but those of a simple cold in the head, the person with a 

 " little sore throat " which is the beginning of an attack of diphtheria, 

 the friend who comes in with uncleansed hands to visit the cook after 

 nursing a sister just put to bed with typhoid fever, the visitor who is 

 "practically all over" whooping cough; these are the dangers against 

 which it is so difficult to guard. 



The term contact is a broad one and covers a wide variety of ways 

 in which infective material may be spread from person to person. 

 There are all degrees between such direct contact, as occurs when one 

 person coughs over another person's hand, and the more remote infec- 

 tion carried by some object which has been recently handled; no abso- 

 lute line can be drawn between what may be infective and what may 

 not. If I handle an apple with infected hands and hand it to you and 

 you eat it, we are dealing with clear and obvious contact. If I put it 

 on the table and you find it and eat it an hour later, the connection is 

 almost as direct, although some of the germs will probably be dead. 

 If twenty-four elapses, most of the infection will be gone ; if two weeks, 

 practically all of it. Objects which are supposed to remain infective 

 after a considerable period of time, are called fomites, and fomites' 

 infection was once held to be an important factor in the spread of 

 disease. The recent discoveries in regard to the rapid mortality of dis- 

 ease germs outside the body have made it clear, however, that objects are 

 only dangerous when they have recently been exposed to fresh infection. 

 The old stories of toys put away in a closet and causing scarlet fever 

 after a lapse of several years are quite apocryphal. Such mysterious 

 cases as were once explained in this fashion are now more reasonably 

 attributed, and very often definitely traced, to direct contact with an 

 unrecognized carrier case. 



