244 BACTERIA, YEASTS, AND MOLDS 



It has been proved recently that yellow fever also is dis- 

 tributed by mosquitoes rather than by direct personal con- 

 tagion. The species of mosquito is different from either of 

 those shown in Fig. 70, and lives only in warm climates. 

 Mosquito netting is the best check for this disease. Yellow 

 fever has been almost wholly stamped out of Havana by 

 simply surrounding the patients with netting, thus pre- 

 venting the mosquitoes from biting them and becoming 

 infected with the germs which they might carry to other 

 persons. It has also been banished from the Panama 

 Canal zone by draining the swamps and closing up the 

 breeding places of mosquitoes ; and the last time that yellow 

 fever invaded the United States it was speedily crushed out as 

 soon as active measures were taken to destroy mosquitoes 

 and prevent their breeding. 



In all truly contagious diseases the parasites have some 



means of leaving the body of the patient. Their methods 



^ of exit are numerous, but are not very diffi- 



^\^ i £^ ^ cult to determine in the case of any particular 



j "if^*,^ disease. Most types of contagious diseases 



. A\J have suggestive symptoms. For example, in 



'**-^ smallpox, there is an eruption of the skin, 



Fig. 7 1. Bacillus ^^ j^. becomes probable at once that this 



of diphtheria. . ^ . 



eruption is a means of elimination of micro- 

 organisms. In diphtheria (Fig. 71) the germs grow in the 

 mouth, clinging to the surfaces inside the mouth and throat, 

 and it is quite evident that the breath, or at all events the 

 forcible breath that comes with coughing, will detach the 

 bacteria from their position in the throat and blow them 

 into the air. In the case of whooping cough the violent 

 paroxysms of coughing are probably a means of eliminating 



