HOW DISEASE GERMS ARE CARRIED 221 



by direct contact. In such diseases as smallpox or scar- 

 let fever, where the infectious material is probably on the 

 skin, contact with the patient would be very likely to 

 infect a healthy individual. Hence, with all diseases of 

 this character, isolation is rightly considered of the greatest 

 importance (see page 241). 



With most diseases, however, other means of transference 

 are more common. The microorganisms are not able to 

 travel of their own accord, and are always carried about by 

 some other agencies, the chief of which are the following. 



Insects. Insects are occasionally the means of carrying 

 infectious material. The relation of the mosquito to 

 malaria and yellow fever has been mentioned, ^^^^ 



and_/f?Vj have a very close relation to the dis- oS'^o 

 tribution of typhoid fever. So close is this rela- ^^o 

 tion that it is now urged that the name typhoid ^'°- 73- ^^• 

 fly should be used. Fleas, also, distribute the tonicplague. 

 bubonic plague, which has recently produced 

 so many deaths in the Old World (Fig. 73). It is quite 

 possible that insects may carry the infection of cholera 

 and some other diseases ; but we know little upon these 

 matters at present. We are thus taught to avoid flies, to 

 shun mosquito bites and flea bites, and, in short, to avoid 

 insects as much as possible. Mosquito netting has, there- 

 fore, an actual sanitary value. 



Larger Animals. Occasionally larger animals transmit 

 infectious microorganisms. It is believed that diphtheria 

 is sometimes carried from the patient in the sick room to 

 another person by cats which wander about the house at 

 will. The bubonic plague, which, fortunately, is as yet 

 rare in this country but which is producing great ravages 



