246 BACTERIA, YEASTS, AND MOLDS 



by direct contact. In such a disease as smallpox, 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, isola- 

 tion is rightly considered of the greatest importance (see 

 page 269). 



With many 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, 



o9^c> ^rA flies have a very close relation to the dis- 



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



FiG. 73. Ba- ^Jqjj ^.jjg^j. j(. jg jjQ^ urged that the name typhoid 



bonic plague, fly should be used. Fleas, also, distribute the 

 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 and 

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

 avoid insects as much as possible. Mosquito netting has, 

 therefore, 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 at will about the 

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

 rare in this country but which is producing great ravages 



