Life and death are determined for millions of human beings by their relationship to the mosquito. Approximately 3,400 species are distinguished, many of which are vectors of one disease or may become so under particular conditions. Blood-sucking mosquitoes act as vectors in the transmission of serious diseases of both man (yellow fever, malaria, filariasis, dengue) and animals (virus encephalitis, yellow fever). Often painful allergic responses follow the salivary injection made by the insect. Methods of control, whereby breeding grounds are destroyed, have often proved difficult and ineffective. The use of chemicals has given rise to new insecticide-resistant populations, leading to the additional cost of research and production of replacement insecticides, with the knowledge that in a few years time these will also have become ineffective. Biological control through natural agents has not been as successful as hoped; but the worst aspect of the problem is that many drugs to treat or to prevent mosquito-borne diseases are no longer having any effect.
Aedes aegypti and the Asian tiger mosquito are two disease carriers.
To kill an infestation of disease-laden mosquitoes in mid-1999, New York city authorities authorised the continuous spraying of Malathion, a known toxic chemical, to kill the mosquitoes. It was deemed necessary to spray the general pollution and city areas to spray to stop the spread of Japanese Encephalitis, the 4th incarnation of the alleged virus borne by the mosquitoes. The strain was first identified as St. Louis Encephalitis, then West Nile Encephalitis, then West Nile-like, and finally Japanese Encephalitis. Each variety of the disease is carried by mosquitoes.