before has fed on the blood of a person sick with this disease during 

 the first 3 days of his illness. . 



2. That yellow fever can be produced under artificial conditions by 

 the subcutaneous injection of blood taken from the general circu- 

 lation of a person sick with this disease during the first 3 days of his 

 illness. 

 . 3. That yellow fever is not conveyed by fomites. 



4. That the Bacillus icteroides Sanarelli stands in no causative 

 relation to yellow fever. 



As all preventive measures are based on the foregoing fundamental 

 propositions, a somewhat more detailed consideration of each is 

 desirable. 



A. — Yellow fever is transmitted, under natural conditions, only by the 

 bite of a mosquito (Stegomyia calopus) that at least 12 days before 

 has fed on the blood of a person sick with this disease during the first 

 three days of his illness. 



The unusual prevalence of insects during some epidemics of yellow 

 fever was noted more than a century ago. It was not until 1848, 

 however, that any suggestion was made as to their etiological con- 

 nection. In that year Josiah C. Nott of Mobile, Ala., reasoning from 

 certain epidemic peculiarities of the disease, expressed it as "probable 

 that yellow fever is caused by an insect or animalcule bred on the 

 ground," and mentioned "mosquitoes, frying ants, many of the 

 aphides" as illustrations of insects whose general habits were such as 

 to fulfill the requirements as transmitters of the disease. At about 

 this time there appears to have prevailed a fairly widespread belief in 

 the existence of some relation between mosquitoes and yellow fever, 

 for Dowler, writing in 1855, states that many persons regarded "any 

 increase in the number of mosquitoes as a 'certain prelude or precursor 

 to a yellow-fever epidemic." 



The first to definitely assert that the mosquito is the medium of trans- 

 mission and to specifically indicate the mosquito concerned was Carlos 

 J. Finlay. In 1881, at a meeting of the Royal Academy of Medical 

 and Physical Sciences of Habana, he stated that three conditions were 

 necessary for the propagation of yellow fever, namely: "(1) The 

 existence of a yellow fever patient into whose capillaries the mosquito 

 is able to drive its sting and to impregnate it with the virulent parti- 

 cles, at an appropriate stage of the disease. (2) That the life of the 

 mosquito be spared after its bite upon the patient until it has a chance 

 of biting the person in whom the disease is to be reproduced. (3)T The 

 coincidence that some of the persons whom the same mosquito hap- 

 pens to bite thereafter shall be susceptible of contracting the disease." 

 During the succeeding twenty years Finlay continued, tenaciously,, to 



