Public health is concerned with threats to the overall health of a community based on population health analysis. Health is defined and promoted differently by many organizations. The World Health Organization, the United Nations body that sets standards and provides global surveillance of disease, defines health as: "A state of complete physical, mental and social well-being and not merely the absence of disease or infirmity." The population in question can be as big as a handful of people or, in the case of a pandemic, whole continents. Public health has many sub-fields, but is typically divided into the categories of epidemiology, biostatistics and health services. Environmental, social and behavioral health, and occupational health, are also important fields in public health.