Health is the condition of a person’s physical and mental functioning and well-being, shaped by biological characteristics, lived circumstances, and the surrounding environment. It is not simply the absence of disease: descriptions of health also consider the ability to perform daily activities, maintain relationships, and participate in society. Medicine addresses health through prevention, diagnosis, treatment, and care, while public health focuses on the conditions and organized activities that affect whole populations.
Definitions and scope
The constitution of the World Health Organization (WHO), which entered into force in 1948, defines health as “a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.” This definition establishes a broad institutional understanding rather than a clinical test that determines whether an individual is healthy.
Other frameworks emphasize functioning and the interaction between people and their environments. WHO’s International Classification of Functioning, Disability and Health distinguishes body functions and structures, activities, participation, and contextual factors. In this framework, a diagnosis alone does not fully describe a person’s daily experience.
Health is therefore measured along several dimensions rather than through a single universal indicator. Someone may have a long-term condition while retaining substantial independence and well-being. Conversely, a person without a diagnosed disorder may experience pain, distress, or limitations that affect daily life.
Physical, mental, and social dimensions
Physical health concerns the functioning of organs and body systems, development, and the capacity to undertake activities. Homeostasis describes the regulation of internal conditions, such as temperature and blood glucose, within ranges compatible with functioning. The immune system contributes to defense against infection, although immune responses can also be involved in illness.
Mental health encompasses emotional and psychological well-being and capacities such as coping, learning, working, and relating to others. It is related to, but not identical with, the absence of a mental disorder. Physical and mental conditions can influence one another through biological pathways, behavior, and social circumstances.
The social dimension concerns relationships, support, inclusion, and participation. Disability involves interactions between health conditions and personal and environmental factors; inaccessible environments can restrict participation independently of the severity of an underlying condition. These dimensions overlap and cannot always be assessed separately.
Determinants of health
Health develops through interacting influences across the life course. Age, inherited characteristics studied in genetics, and exposures during development affect susceptibility to particular conditions. Their effects are not necessarily deterministic: biological predispositions often interact with environmental circumstances.
Social determinants of health include the conditions in which people are born, grow, live, work, and age. Income, housing, working conditions, and access to education influence exposure to hazards and the resources available to respond to them. Nutrition, physical activity, sleep, tobacco exposure, and alcohol consumption also affect health, but these patterns are themselves shaped by social and commercial environments.
Environmental conditions include air quality, sanitation, food safety, and access to safe water. Climate change affects health through pathways including extreme heat, changing infectious-disease risks, and disruption of food and water supplies. Access to timely, appropriate care is another determinant, but health outcomes cannot be explained by healthcare provision alone.
Population health and prevention
Epidemiology studies the distribution and determinants of health-related states and events in populations. It examines patterns across time, place, and groups, using methods that distinguish observed associations from evidence about causes.
Prevention is commonly classified into three levels. Primary prevention acts before a disease develops, for example through sanitation or the use of a vaccine. Secondary prevention seeks earlier detection and intervention. Tertiary prevention seeks to limit complications and functional losses after disease is established.
Health promotion addresses people’s capacity to influence health and the conditions supporting it. It includes organizational and community approaches, not only information directed at individuals. Preventive interventions require evaluation: their effects depend on the condition, the population, and the balance of benefits and harms. Screening, in particular, is not equivalent to diagnosis and can produce false-positive or false-negative results.
Measurement and evidence
Population health indicators include mortality rates, disease incidence and prevalence, and life expectancy. Incidence describes new cases over a specified period; prevalence describes existing cases in a population. Life expectancy summarizes mortality conditions and does not predict an individual’s lifespan.
Disability-adjusted life years combine years lost through premature death with years lived with disability. Surveys and clinical assessments also measure symptoms, functioning, and quality of life. Different indicators answer different questions, and averages can conceal substantial differences between population groups.
Health research uses statistics to estimate effects and uncertainty. A randomized controlled trial can evaluate interventions under specified conditions, while observational research is important for exposures and circumstances that cannot readily be randomized. Findings require interpretation in light of study design, measurement quality, and applicability.
Healthcare systems and rights
A health system includes the people, institutions, resources, and activities organized to maintain or improve health. Its functions encompass prevention, treatment, rehabilitation, and palliative care, supported by financing, workforce development, information, and governance.
Universal health coverage means access to needed quality health services without financial hardship. It concerns both service availability and financial protection. Within international human rights frameworks, the right to health refers to the highest attainable standard of health, not a guarantee of being healthy. It includes access to healthcare and underlying conditions such as safe water, sanitation, and adequate housing.