On the other hand, there are places wholly outside of the normal range of _Aëdes calopus_ where the disease has raged. Such are New York, Boston, and even Philadelphia, which have suffered notable epidemics. These outbreaks have been due to the introduction of infected mosquitoes during the heat of summer, when they have not only conveyed the disease but have found conditions favorable for their multiplication. Or, uninfected mosquitoes have been thus accidentally brought in and developed in large numbers, needing then only the accidental introduction of cases of the disease to start an epidemic. Methods of control of various diseases have been revolutionized by the discovery that they were insect-borne, but in no other case has the change been as radical or the results as spectacular as in the case of yellow fever. The "shot-gun quarantine," the sufferings and horrors, the hopelessness of fighting absolutely blindly have given way to an efficient, clear-cut method of control, based upon the knowledge that the disease is carried from man to man solely by the mosquito, _Aëdes calopus_. The lines of defense and offense are essentially as follows: In the first place, when a case of yellow fever occurs, stringent precautions must be adopted to prevent the infection of mosquitoes and the escape of any already infected. This means that the patient must be removed to a mosquito-proof room, or ward beyond reach of the insects, and that the infected room must be thoroughly fumigated at once, to kill the mosquitoes hiding within it. All cracks and openings should be closed with strips of paper and fumigation with burning sulphur or pyrethrum carefully carried out. It should be remembered that if the first case noted is that of a resident rather than imported, it means that the mosquito carriers became infected more than two weeks before the case was diagnosed, for as we have seen, the germ must undergo a twelve-day period of development within its insect host. Therefore a careful search must be made for mild cases which, though unrecognized, may serve as foci for the spread of the disease. In face of a threatened epidemic one of the most essential measures is to educate the citizens and to gain their complete coöperation in the fight along modern lines. This may be done through the schools, the pulpit, places of amusement, newspapers and even bulletin boards. Emphasis should be placed on the necessity of both non-immunes and immunes using mosquito curtains, and in all possible ways avoiding exposure to the mosquitoes.