It is very surprising and impressive to see how rapidly such a system will free a city of mosquitoes, and how after a few months of such work you cease to be annoyed by them. In yellow-fever work this system of destroying mosquito larvæ is the essential; everything else is secondary to it. In the built-up portions of a city such as Havana, caring for the cisterns, water barrels and containers is the essential work, but as you approach the suburbs, pools and puddles become more frequent, and this character of mosquito breeding-places becomes more important than containers. Wherever possible these breeding-places should be drained, though oiling in this class of work has a very useful field. In the suburbs, in these pools and puddles, the anopheles, the malarial mosquito, becomes common and this disease has to be looked after. We had fifty men engaged in this work, under a different set of inspectors from those doing the stegomyia work. This was made necessary, as the men doing the anopheles work were occupied almost entirely in the suburbs of the city. The details I will describe in another place. CHAPTER VI THE RESULTS ACCOMPLISHED IN HAVANA From the account already given of yellow fever it is evident that the parasite has to be introduced into a locality, either in the body of a human being sick of yellow fever, or in the body of a female stegomyia mosquito which has become infected by the parasite. It is evident that if the health authorities can keep out these two sources of infection, yellow fever can never occur in that locality. With this object in view we established a modern quarantine at Havana. Any ship coming in with yellow fever aboard was placed in quarantine. The vessel itself was fumigated so as to kill all the mosquitoes. This rendered the ship safe. All the non-immune crew were taken off, carried to quarantine station, where they were cared for during a period of six days. If no sickness developed during this period, it was concluded that the infected mosquitoes aboard the ship had not bitten these non-immunes up to the time of their departure. They were therefore released and allowed to return. The immunes were not considered liable to yellow fever, even if they had been bitten by the infected mosquitoes.