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Still, other factors are beyond both individual and group choices, such as genetic disorders. The population in question can be as small as a handful of people or as large as all the inhabitants of several continents . Public health has many sub-fields, but typically includes the interdisciplinary categories of epidemiology, biostatistics and health services. Environmental health, community health, behavioral health, and occupational health are also important areas of public health. From ancient times, Christian emphasis on practical charity gave rise to the development of systematic nursing and hospitals and the Catholic Church today remains the largest non-government provider of medical services in the world. This is intended to ensure that the entire population has access to medical care on the basis of need rather than ability to pay.

• For example, it is estimated that life expectancy has increased for