We define the public health sector to include any organization, program or activity that works to create the conditions in which people can live healthy lives, including activities to prevent disease and injury and promote health for the population at large. They include governmental public health agencies working at local, state, and federal levels, as well as nongovernmental organizations that contribute to the performance of core public health functions. A defining feature of public health systems is their focus on actions designed to protect and improve health at a population level rather than purely at an individual level through delivery of personal health services.  Public health systems implement activities to protect populations from communicable diseases, to prevent chronic disease risks and injuries, to promote healthy behaviors, and to reduce environmental health risks in the air, water, food, and built environment.