What is present in nearly every U.S. community, performs myriad services from the routine to the life saving on a daily basis, responds to every disaster, and functions 24 hours a day every day of the year? The answer, of course, is the nation's $1.8 trillion public health and healthcare system. Protection of this system's vast infrastructure has assumed increasing urgency since September 11, and there are at least two reasons for this. The first is that this sector must respond to every conceivable event involving risks to human life, including those traditionally within the purview of public health, so its ability to respond to these events must be preserved. The second is that elements of the sector itself face increasing threats to facilities, information systems, and workforces. These reasons alone warrant greater emphasis on protective programs than may have seemed necessary in the past, and the public health and healthcare sector should recognize that it must now understand critical infrastructure protection as well as it does healthcare management.