WHAT IS PHYSICAL LIFE 



and this infection continues its same nefa- 

 rious ways till now. Yellow fever remains 

 the same yellow fever in every respect as 

 the first historic description given of it. 



This mention of smallpox and yellow 

 fever justifies our alluding to the contrasts 

 between the living agents of these two in- 

 fections, if only further to emphasize the 

 specific differences which distinguish them 

 from one another. The yellow fever germ 

 can go nowhere except as his mosquito 

 nurse carries him. Moreover, he cannot 

 endure the slightest touch of frost, though 

 that may not be fatal to his mosquito. 

 He has never been known to ascend moun- 

 tains. The smallpox agent, on the other 

 hand, can be carried by anything, as was 

 illustrated on one occasion by an official 

 visit of mine to a smallpox hospital, ac- 

 companied by a confrere. There was but 

 one smallpox patient in the building, and 

 we were careful not to come near enough to 

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