1 



The next step was to enquire into the 

 question of insect transmission. A com- 

 mon domestic mosquito of Cuba is the 

 Stegomyia fasciata, a strikingly marked 

 insect of voracious habits, and suspicion 

 had already attached itself to it. Both 

 sexes of it suck blood, and it is probably 

 the most widely distributed of all mos- 

 quitoes, being found practically all 

 through the tropical world, and also in 

 many sub-tropical localities. Twelve 

 plucky young Americans, two of whom 

 had taken part in the experiments with 

 clothing previously described, v©lunteered 

 as subjects. All these were living under 

 exactly the same conditions as scores of 

 other non-immune residents of the camp, 

 and the two I have mentioned had been 

 free for 30 days from their self-imposed 

 imprisonment in the infected clothes hut. 

 All were deliberately subjected to the 

 bites of specimens of this mosquito, which 

 had been previously allowed to bite early 

 cases of Yellow fever. After periods of 

 from three and a-half to six days from 

 bemg bitten, ail except two developed true 

 attacks of Yellow fever. 



One experiment in the batch was of 

 especial interest. A newly - erected 

 building was divided into two parts 

 by fine-wire screens. In one part were 

 let loose 15 infected mosquitoes, and 

 in the other half, separated only by a 

 wire screen, two non-immune men lived 

 and slept. The volunteer subject entered 

 the part in which were the mosquitoes on 

 three occasions of about twenty minutes 

 each, and was freely bitten on 

 each occasion. On the fourth day afcer 

 his visit he developed a severe attack of 

 Yellow fever, while the two men in the 

 part on the otheL' side of the gauze screen 

 continued for 18 nights to sleep in and 

 breathe the common atmosphere of the 

 room without any ill effect. This was 

 pretty fair evidence even for the scientific 

 mind that the mosquitoes were the con- 

 veyors of the disease, and subsequent ex- 

 periments proved this to be so beyond all 

 doubt. \ 



The organism which is the actual 

 cause of Yellow fever has not been 

 discovered, probably because it is so 

 minute as to escape the highest 

 powers of the microscope. This is the 

 more likely since, as in the case of 

 malaria, the injection of blood from an 

 acute case will produce the disease in a 



non-immune subject, but, unlike malaria, 

 the blood remains similarly infective after 

 it has been passed through the finest Berk- 

 feld filter, thus proving the extreme 

 minuteness of the organism, since such a 

 filter will stop the smallest of known bac- 

 teria. That it IS not a bacteria, but an 

 organism more of the type of the malarial 

 organism is very probable from the fact 

 that the infection cannot be obtained by 

 the mosquito from the blood, except in 

 the first three days of illness, and the 

 mosquito does not become capable of in- 

 fecting fresh persons until 12 days or more 

 after it has fed upon a case at this par- 

 ticular stage. The fact that an outbreak 

 of Yellow fever took a]Jfortnight or so to 

 light up after the introduction of a case 

 into a locality had long been noticed, and 

 this explained the reason thereof. 



Having obtained these facts, the 

 battle against Yellow fever assumed 

 a new aspect. Huge sums had 

 been spent during previous years in 

 indiscriminate disinfecting of articles 

 which had been in contact with Yel- 

 low fever patients— quite harmless as 

 we now know — and in other expensive 

 measures, absolutely without avail. The 

 campaign was now begun by a wholesale 

 destruction of mosquitoes and their 

 breeding places in swamps and puddles, 

 and by the careful screening of 

 Yellow fever patients for the three 

 first dangerous days, from mosquito bite 

 in order to prevent the loading up of fresh 

 winged carriers of disease. Quarantine 

 of cases was given up — it had been en- 

 forced for many years without avail, — 

 and the only quarantine order was that 

 of the mosquito net. The resuU was 

 amazing. In ninety days from the com- 

 mencement of operations Havana was 

 freed from Yellow fever for the first time 

 in 140 years. Repeated introductions 

 from without, where the measure? 

 were not in force and where Yellow 

 fever was raging unchecked, as it had 

 done for decades past at that season, were 

 promptly stamped out, and the fact was 

 realised that although every effort 

 known to sanitary science had been 

 put forth without avail up to March, 1901, 

 the change effected in the measures by 

 the recognition of the mosquito as 

 the carrying agent had enabled the au- 

 thorities to obtain in 13 weeks a 

 result which had defied the efforts of 



