HARI SREENIVASAN:

Revelations of safety and security problems put Dr. Thomas Frieden under the microscope at a House hearing. The head of the Centers for Disease Control and Prevention was called to account after more than 80 CDC lab workers were exposed to live strains of anthrax last month in Atlanta.

The agency has also acknowledged that it mistakenly shipped the avian flu virus to outside labs. Separately, several 60-year-old vials of smallpox, some with still viable strains, were found at the National Institutes of Health in Bethesda, Maryland.

No one was sickened in the incidents, but Frieden conceded:

DR. THOMAS FRIEDEN, Director, Centers for Disease Control and Prevention: The fact that it appears that no one was harmed and that there were no releases does not excuse what happened. What happened was completely unacceptable. It should never have happened.