HEALTH AND DISEASE 333 



Steam sawmill, where there was a pond of like offensive water, were taken 

 with violent fevers." Dr. Rush in describing the outbreak of the 1793 epi- 

 demic in Philadelphia says: "Upon inquiry, it appears that the first per- 

 sons who died with this fever . . . had been previously exposed to the 

 atmosphere of the wharf." As the mosquito breeds in still water, here was 

 another clue pointing to it as a carrier of yellow fever. 



THE THIRD CLUE 



In the high and dry parts of a city the disease was not contagious. In 

 many epidemics people from low-lying sections fled to the higher part of 

 the city or to the country districts. Although many of these people came 

 down with yellow fever after they had left their homes, the disease did 

 not spread to other people in the new neighborhood. 



This clue pointed to the thought that yellow fever must be carried in 

 some way other than directly from one person to another. This was the 

 conckision arrived at by a great many intelligent observers, but the only 

 explanation they could give was that the disease must be present in the 

 air of certain districts and not in others. 



THE FOURTH CLUE 



Another clue strengthened the idea that the disease was air-borne. Some 

 people noticed that the fever spread in the direction of the prevailing wind. 

 Whenever the wind blew strongly in a certain direction, yellow fever 

 broke out in its path. When the air was still, the infection was content to 

 pay its calls in the houses of an already infected neighborhood. As the 

 mosquito is a great lover of home, and never travels far unless it gets a free 

 ride on the wind, or on a ship, this clue explains why yellow fever spread 

 so quickly in narrow streets, and broke out at a distance from the wet low- 

 lying districts of a city only when the villain of the drama was carried there 

 by wind. 



THE FIFTH CLUE 



Yellow fever flourished when the weather was hot but was stamped out 

 by frost. A4osquitoes, also, are active in hot weather and disappear after 

 a frost. Here was another important clue, but it didn't mean anything ex- 

 cept that "heat was a very common exciting cause of the disorder," until 

 suspicion was thrown on the mosquito. It is easy to explain facts that seem 

 mysterious as soon as the villain of a detective story is uncovered. Then 

 it seems strange that the important clues, which pointed to the guilty person 

 as clearly as a signboard points out a road, could have been misunderstood. 



Anyone who has studied the life history of the mosquito can see how 

 the spread of yellow fever tallies with the mosquito's habits. But it is one 

 thing to suspect a villain, and another thing to prove the suspicion to be 

 true. Someone had already suspected that the mosquito carries yellow 



