During such periods the latent parasites, which have been perhaps for months lying hidden in the internal organs, will reappear in the blood in large numbers, and there are always a sufficient number of such cases of relapse persisting through the naturally malaria-free season, to suffice to infect the early broods of mosquitoes that come to maturity with the advent of the rains. This shows that the curing of all cases of malaria is matter by no means confined to the interests of the patient himself, but is of great importance to the entire community of which he is a member; for, despite the fact that the disease is only indirectly transferable through the agency of the mosquito, a case of malaria is as great a danger to his fellow-men as one of any other infectious disease, and should, as far as possible, be dealt with in the same way; only the problem of isolation is in this case easier than usual, as it suffices to prevent mosquitoes getting access to the sick person; and in a properly fitted mosquito-proof house or hospital, malaria cases may quite safely be allowed to mix freely with the healthy, as well as with persons affected with other diseases. Relapses in persons chronically infected with malaria may occur at any time of the year, but the fresh infections, which always form the bulk of the cases met with, can only happen as the result of a tolerably recent bite by an infected mosquito; and as about a couple of weeks are required for the maturing of the parasite within the insect, and a further period of incubation is necessary within the human subject before the parasites attain sufficient numbers to produce a definite constitutional reaction, fever does not as a rule commence in earnest until some three weeks or a month after the break of the wet season. Once the process is started, infected mosquitoes and malaria-stricken men increase in numbers rapidly, and as numbers of infected mosquitoes survive for a considerable period after the cessation of the breeding season, the prevalence of malaria continues until the advent of cold weather puts a period to the possibility of the survival of the parasite within the insect organism. Hence it is generally quite possible to give a fair idea of the monthly distribution of rainfall in any warm climate from the returns of sickness and mortality, and _vice versâ_. The natural history of the living organisms that are concerned in the propagation of malaria has been described with some detail, as suitable measures for the prevention of the disease must needs be based on a fairly competent knowledge of the subject, and is the more necessary as it is hardly possible to give any “rule of thumb” directions; because the circumstances on which the prevalence of malaria depends vary so greatly in different localities that, to ensure success, our operations must necessarily be modified in each case to meet local conditions.