A disease is a condition involving disturbed structure or functioning in an organism, recognized through characteristic clinical features, biological abnormalities, or diagnostic criteria. In human medicine, diseases are described by their manifestations, causes, affected body systems, and course. Disease does not necessarily produce immediately noticeable symptoms: pathological changes may precede recognizable illness, and some conditions remain clinically inapparent. Identifying and classifying disease provides a basis for clinical care, research, and public health. (icdcdn.who.int)
Clinical features and terminology
Diseases may be recognized through symptoms reported by a person, signs found during examination, and abnormalities detected by laboratory tests or imaging. Diagnosis is the process of identifying a disease or other condition using this evidence. A medical history, physical examination, blood tests, imaging studies, and tissue biopsies may contribute to diagnosis; the combination used depends on the suspected condition. (cancer.gov)
A syndrome is a group of symptoms or conditions that occur together and suggest a disease or an increased likelihood of developing one. The term describes a recognizable association of findings rather than necessarily specifying a single cause. Disease identification therefore involves more than naming an isolated complaint: clinicians consider the pattern of findings and the available evidence about its origin. (cancer.gov)
Causes and mechanisms
The study of disease causation is called etiology. In epidemiology, a traditional model of infectious disease distinguishes an agent, a susceptible host, and an environment that permits exposure. Agents include viruses, bacteria, parasites, and other disease-producing organisms. Exposure alone does not always produce disease: the agent’s properties and dose, together with the host’s susceptibility, influence the outcome. (archive.cdc.gov)
Host factors include genetic composition, age, nutritional status, anatomical characteristics, and the functioning of the immune system. Environmental factors include sanitation, crowding, climate, and access to health services. These interactions explain why the same exposure can have different consequences in different individuals or populations. (archive.cdc.gov)
Many noninfectious diseases have multiple contributing causes rather than one sufficient cause. Genetic, physiological, environmental, and behavioral factors interact in major noncommunicable conditions. Tobacco exposure, air pollution, raised blood pressure, and elevated blood glucose are among the established risk factors for these diseases. A risk factor increases the likelihood of disease but does not mean that every exposed individual will develop it. (who.int)
Classification
Diseases can be grouped by cause, anatomical location, clinical features, or other established characteristics. Broad categories include infectious and parasitic diseases, neoplasms, and disorders affecting particular organs or systems. These categories organize medical information but represent different dimensions of disease: an anatomical description and a causal description may both apply to the same condition. (icdcdn.who.int)
Infectious diseases involve biological agents and susceptible hosts. Transmission can occur through different routes, with reservoirs in humans, other animals, or the environment. Infection is not synonymous with symptomatic disease: an infectious agent may invade a host without producing clinically apparent illness. (archive.cdc.gov)
Noncommunicable diseases are not transmitted between people through infectious transmission. Major groups include cardiovascular diseases, cancers, chronic respiratory diseases, and diabetes. These conditions tend to last a long time. Cancer itself comprises multiple diseases characterized by uncontrolled abnormal cell division and the potential to invade nearby tissues or spread elsewhere in the body. (who.int)
The World Health Organization maintains the International Classification of Diseases (ICD). It standardizes the recording and comparison of diseases, related health problems, and causes of death. ICD-11 was adopted in 2019 and came into effect on January 1, 2022; implementation proceeds through national health-information systems. (who.int)
Natural history and outcomes
The natural history of a disease is its progression over time without treatment or other intervention. A typical framework distinguishes susceptibility, subclinical disease, clinical disease, and eventual outcomes. During a subclinical stage, pathological changes are present without noticeable symptoms. In infectious diseases, the interval between exposure and symptom onset is generally called the incubation period. (archive.cdc.gov)
Clinical expression varies from mild to severe, and some disease processes never become visibly symptomatic. Outcomes can include recovery, persistent impairment, disability, or death. Preventive and therapeutic measures can alter the course, so an observed clinical outcome is not necessarily the outcome that would have occurred without intervention. (archive.cdc.gov)
Population measurement and prevention
Population studies distinguish incidence, which concerns new cases arising during a specified period, from prevalence, which concerns existing cases in a population. Mortality measures deaths, while morbidity concerns illness. These measures support comparisons between populations and over time, although differences in age structure, diagnostic practice, and case definitions require attention. (cdc.gov)
Disease prevention is commonly organized into three levels. Primary prevention seeks to prevent disease from developing, including through vaccination and reductions in harmful exposures. Secondary prevention concerns early detection and intervention where these improve outcomes. Tertiary prevention addresses established disease, seeking to reduce complications and support rehabilitation. (applications.emro.who.int)
Clinical care spans diagnosis, treatment, rehabilitation, and palliative care. Population approaches also address social determinants of health, including the conditions that shape exposure and access to services. Within a health system, these activities form a continuum rather than separate responses limited to the moment when symptoms appear. (who.int)