Public health has been defined as "the science and art of preventing disease”, prolonging life and improving quality of life through organized efforts and informed choices of society, organizations, public and private, communities and individuals. Analyzing the determinants of health of a population and the threats it faces is the basis for public health. The public can be as small as a handful of people or as large as a village or an entire city; in the case of a pandemic it may encompass several continents. The concept of health takes into account physical, psychological, and social well-being. As such, according to the World Health Organization, it is not merely the absence of disease or infirmity and more recently, a resource for everyday living.Public health is an interdisciplinary field. For example, epidemiology, biostatistics, social sciences and management of health services are all relevant. Other important subfields include environmental health, community health, behavioral health, health economics, public policy, mental health, health education, occupational safety, gender issues in health, and sexual and reproductive health.