ORANGE PARK, Fla. – Oct. 19, 2009, started like any other day for Somer Thompson. The 7-year-old woke up, ate breakfast and rushed off to school with her twin brother and older sister.

Thompson named Julie L. Gerberding director of the CDC. Among public health experts, it was a widely praised appointment. During the anthrax attacks of 2001, Gerberding had stood out as a ...