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tinuous; but in most of these eases it was accompanied by diarrhoea (some- 

 times mixed with blood). This instance, at any rate, shows that Koch's 

 assertion, that "where there are no mosquitoes there is no malaria" is al- 

 together too absolute. In those camps, I believe the propagation must have 

 been effected through the flies (of which there was a great abundance). 

 These insects, in spite of all precautions, had ample opportunities of picking 

 up, from the discharges of the malaria patients, not only the malaria 

 parasites contained in the extravasated blood but also some infectious 

 intestinal germ, with both of which organisms they may have contaminated 

 the food and beverages used by the men who subsequently showed signs of 

 the double infection. A yellow fever epidemic occurring under similar cir- 

 cumstances, in the absence of yellow fever mosquitoes, might be so readily 

 reconciled with my theory about that disease, which is founded upon more 

 definite and more exclusive arguments than those recorded in connection 

 with the malaria infection. The following instance may serve to illustrate 

 my meaning. In the capital of Mexico, and in other districts of similar al- 

 titude above the sea level, Mexicans who never have visited the lowlands 

 have no immunity whatsoever against yellow fever a sure proof that no 

 epidemics of that disease ever occur in that part of the country. It sometimes 

 happens, however, that a resident of the capital takes the the infection by 

 going to Vera Cruz, though the disease may not declare itself until his 

 return to the capital. In such cases, the yellow fever will run its usual 

 course with the same symptoms and prognosis as if the patient had 

 remained at Vera Cruz; with this difference only; that in Vera Cruz 

 other susceptible persons might readily have caught the infection from 

 him, whereas in Mexico the disease is never propagated. If the infection 

 could be transmitted through contact with the patient or his secretions, 

 by inhaling his emanations in the sick room, or by the use of contaminated 

 food or beverages, there would be no imaginable reason why the disease 

 should not be transmitted at Mexico as well as at Vera Cruz. Such not 

 being the case, we must infer: first that a factor which is necessary for 

 the transmission is present at Vera Cruz, but is absent from Mexico ; and 

 second, from the circumstance of the disease not being transmissible through 

 the forms of exposure enumerated above, that the yellow fever germ is 

 pathogenous only when introduced in a less trivial manner, probably by 

 inoculation under the epidermis or even directly into a blood vessel. Hence 

 my theory of the mosquito. 



New Mosquito Theory. — My original mosquito theory, however in 

 view of the facts brought to light by Dr. T. H. Smith in his admirable 

 demonstration of the transmission of the Texas fever through the agency 

 of the cattle tick, requires now to be somewhat modified, so as 

 to include the important circumstance that the faculty of transmitting 

 the yellow fever germ need not be limited to the parent insect, dir- 

 ectly contaminated by stinging a yellow fever patent (or perhaps by 



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