The end of the summer of 1999 turned out to be a scary one for the U.S. In New York state, an outbreak of viral encephalitis inexplicably took hold; seven people died between Sept. 5 and Oct. 19 [source: CDC]. Viral encephalitis is caused by an infection of the central nervous system, where the tissue surrounding the brain and spinal cord -- the meninges -- becomes inflamed and alters their function, leading in some cases to coma and then death.
Patients suffering from acute, advanced encephalitis suffer from symptoms like difficulty breathing, seizures, memory loss and behavioral changes like the development of hydrophobia -- the fear of the sight of water [source: PDR].
As the outbreak progressed, investigators from the Centers for Disease Control and Prevention (CDC) discovered its origin. The patients suffered from an infection of the West Nile virus (WNV). An arcane virus that had plagued only Africa, Asia and the Middle East had made the jump to North America. There's no inoculation against it, and since West Nile is transmitted most commonly through mosquitoes, infection can only be prevented by thwarting the insects from biting.
While only about 1 percent of WNV cases develop into encephalitis, Americans reached for the insect repellent in droves that summer. Those folks stopped short of spraying it, however [source: Grady]. West Nile appeared at a time when the country was increasingly concerned about the safety of N,N-diethyl-m-toluamide (DEET), a common active ingredient in bug spray. DEET was created by the U.S. Department of Agriculture (USDA) in 1946 for military use and sold to consumers in the late 1950s [source: RAND]. Over the years, reports of children suffering seizures and other illnesses joined widely covered stories of a man who'd suffered a psychotic break after using too much bug spray and a woman who'd suffered a heart attack after applying bug spray before gardening [source: CDC].
Mosquitoes bearing the West Nile virus descended on a population that was increasingly interested in natural remedies, which came with an attendant suspicion of all things chemical. Chemical researchers and manufacturers were forced to admit that they don't really know how DEET works (the prevailing theory is that it masks humans from mosquitoes' perception). This put people who feared West Nile and DEET equally in quite a pickle.
Of special concern was the use of bug sprays by children and pregnant women. A rumor spread that DEET proved the most dangerous to these two groups, and federal agencies and researchers began to investigate.