Public health is, according to the institute of medicine's definition -- public health is what we, as a society, do collectively to assure the conditions for people to be healthy -a field that is activist in nature and concerned with the social nature of health.
Public health is concerned with groups (of both sick and well people) ranging anywhere from the smallest of communities to the entire globe.
Public health is a social and governmental process that is essentially pragmatic in nature, an expression of democracy itself, not a 'thing' guaranteed in a structure of law-based rights.
Public health is an enactment of social justice. Social justice is in fact the foundational value of public health.
Public health is concerned with preventing disease by promoting healthy behavioral changes and by reducing exposure to environmental risks (unsafe water) and social risks (gender discrimination).
Public health is concerned both with prevention (for healthy people) and with ensuring access to treatment (for unwell people).
Public health is a modernist progressive enterprise, drawing on the available expert knowledges, technologies, means of calculation, goal-orientation, enumeration, and surveillance to further the good of members of the public by improving their health status.
Public health is concerned with ensuring that the health systems are in place to provide care and treatment for people that need it via capability (i.e. Health service planning, governances, leadership, information management) and capacity (i.e. Finances and human resources).