(4) The process of multiplication of the parasites within the human body is by simple division, or non-sexual breeding of the single cell of which each parasite consists; but by a well-known law of the life-history of this class of animalcules this method of multiplication cannot continue indefinitely without the intervention of a period of sexual multiplication; and this can not occur within the human subject under any circumstances, but only in the bodies of certain species of mosquitoes; so that the disease always tends to wear itself out, provided the strength of the patient holds out sufficiently long; and the parasites are unable to find their way into the blood of other human beings by natural processes, unless mosquitoes of certain special species be present. (5) If, however, a human being infected with malaria be bitten by a mosquito of the appropriate sort, the parasites, sucked into the stomach of the mosquito along with its meal of blood, undergo further development into distinct male and female animalcules, whose union gives birth to myriads of germs which, although incapable of further multiplication within the organism of the insect, find their way into its salivary glands, which are the organs in which the irritating poison of the mosquito is elaborated, and so are necessarily inoculated into the tissues of any human being whom the mosquito may bite, with the result that a new victim becomes infected, and the chain of events commences anew. (6) The malarial parasites can exist in the mosquito only within certain limits of temperature, and hence the disease is not found in countries where the maximum summer temperature is less than 76° F., or in places so hot that the temperature rises above 86° F. for any length of time, and the comparative healthiness of many hot continental climates is due to the fortunate circumstance that a period of excessive dry heat follows immediately on the cold season. In considering the measures of prevention detailed below, it must be remembered that their importance is enhanced by the fact that not only malaria, but also the blood-worm disease already alluded to, or filariasis, and yellow fever, have also been shown to require the intervention of mosquitoes for their transmission from man to man, and that although not carried by mosquitoes, the germs of sleeping sickness are also probably conveyed by a winged insect, so that many of the measures detailed below will have a certain value also in the prevention of the last-mentioned disease.