Last fall, something peculiar began to happen at more than two dozen elementary and middle schools scattered across the country. Suddenly, groups of children started breaking out with itchy red rashes that seemed to fade away when the children went home -- and to pop up again when they returned to school. Frustratingly for the federal, state and county health officials who were working to explain this ailment, it did not conform to any known patterns of viral or bacterial illness. The children had no other symptoms: no fever, no runny noses, no headaches or joint pain or respiratory complaints....