One of the Bush administration's principal advisers on bioterrorism and other public health emergencies has resigned.

Stewart Simonson, assistant secretary for public health emergency preparedness, told the president in a resignation letter Monday that he had accomplished what he had set out to do, and it was time to move on.

Some Democrats, and a key Republican, Rep. Tom Davis of Virginia, had been critical of Simonson's work, saying it would be better to have a medical expert serve in his position rather than a lawyer. Davis is chairman of the House Government Reform Committee...