Insects that are alternate hosts for a disease are essential in its transmission.
Insects can be so numerous that, once transmission of a disease has been established in a particular area, especially in the tropics and areas where sanitation is poor, many or most of the members of the human communities in that area will be affected. Perhaps the best example is provided by malaria in areas where eradication of the disease has not as yet been achieved or is in its early stages. In many parts of west Africa, there is probably no child who has not been infected once maternal immunity has been lost. Consequently, there are areas where as much as 10% of the mortality and a major part of the morbidity occurring among young children may be due to malaria. Such a high incidence of disease can be accounted for when it is realized that many anopheline mosquitoes may feed on a person every night, thus making it almost certain that at least one of them will carry the malaria infection.