It is clear from what has been said regarding the life cycle of the malarial parasite that the mosquito is harmless if not itself diseased. Hence malarial-bearing species may abound in the neighborhood where there is no malaria, the disease being absent simply because the mosquitoes are uninfected. Such a locality is potentially malarious and needs only the introduction of a malarial patient who is exposed to the mosquitoes. It is found that such patients may harbor the parasites in their blood long after they are apparently well and thus may serve as a menace, just as do the so-called typhoid carriers. In some malarious regions as high as 80-90 per cent of the natives are such malaria-carriers and must be reckoned with in antimalaria measures. Based upon our present day knowledge of the life cycle of the malarial parasite the fight against the disease becomes primarily a problem in economic entomology,--it is a question of insect control, in its broadest interpretation. [Illustration: 129. Larva of Anopheles. After Howard.] The lines of defence and offence against the disease as outlined by Boyce (1909) are: 1. Measures to avoid the reservoir (man): Segregation. Screening of patients. 2. Measures to avoid Anopheles: Choice of suitable locality, when possible. Screening of houses and porches. Sleeping under mosquito nets. 3. Measures to exterminate the Anopheles: Use of natural enemies. Use of culicides, oiling ponds, etc. Drainage and scavenging to destroy breeding places. Enforcement of penalties for harboring larvæ or keeping stagnant water. Educational methods. 4. Systematic treatment with quinine to exterminate the parasites. MOSQUITOES AND YELLOW FEVER Yellow fever was until recently one of the most dreaded of epidemic diseases. It is an acute, specific and infectious disease, non-contagious in character but occurring in epidemics, or endemics, within a peculiarly limited geographical area. It is highly fatal, but those who recover are generally immune from subsequent attacks. It is generally regarded as an American disease, having been found by Cortez, in Mexico, and being confined principally to the American continents and islands. It also occurs in Africa and attempts have been made to show that it was originally an African disease but there is not sufficient evidence to establish this view.