More than two decades ago, a little-known mercury-based product, thimerosal, was approved by the Food and Drug Administration to be used as a vaccine preservative. Since then, many parents and scientific studies have questioned whether it is responsible for the current epidemic of neurological and developmental disorders, such as learning disabilities and autism. In July 1999, U.S. health agencies and the American Academy of Pediatrics (pdf) reduced or eliminated thimerosal from vaccines as a "precautionary measure." Yet up to 25 micrograms of thimerosal remain in several versions of the influenza vaccines recommended this flu season, including those administered to pregnant women, infants, and children, and it is still present in some multi-dose vial vaccines, despite hundreds of studies showing thimerosal is harmful.