Updated

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.

Simonson joined the Department of Health and Human Services in August 2001 and served as general counsel before becoming assistant secretary.

During his time at the department, the first vaccine against the H5N1 bird flu virus was developed and the administration stockpiled enough smallpox vaccine to immunize every person in America, plus an additional 20 million doses for the World Health Organization.