Roundtable on public health preparedness in the 21st century : hearing of the Subcommittee on Bioterrorism and Public Health Preparedness of the Committee on Health, Education, Labor, and Pensions, United States Senate, One Hundred Ninth Congress, second session, on examining public health preparedness in the 21st century, focusing on a national public health infrastructure with real-time situational awareness, and responding to threats by terrorism or natural disasters, March 28, 2006.