Epidemiology is the study of the distribution (who, where, when) and determinants (why) of health-related states and events in specified populations, and the application of that study to control health problems. The unit of concern is the population, not the individual client. Public health nurses use epidemiology to find groups at risk, set priorities, plan programs, and judge whether programs work.

Two branches

Uses of epidemiology

• Identify causes and risk factors of disease
• Describe the natural history of disease
• Measure the health status and burden of disease in a community (community diagnosis)
• Plan and evaluate health services and programs
• Complete the clinical picture for practice (for example, the usual course of an illness)

Basic terms

• Risk factor: a characteristic or exposure associated with a higher chance of disease (smoking for lung cancer). A protective factor lowers the chance.
• Determinants of health: biology, individual behavior, social and physical environment, health services, and policy. Healthy People 2030 frames the social determinants in five domains: economic stability, education access and quality, health care access and quality, neighborhood and built environment, and social and community context.
• Population at risk: the people who could develop the condition (for example, only women are at risk of cervical cancer). It is the correct denominator for risk and rate calculations.
• Endemic: constant usual level of a disease in an area. Epidemic (outbreak): occurrence clearly above the expected level. Pandemic: an epidemic spread over several countries or continents. Sporadic: occasional, irregular cases.

Counts, ratios, proportions, and rates

Worked example — ratio vs. proportion. A clinic has 240 women and 160 men enrolled in a hypertension program.

• Female-to-male ratio = 240 ÷ 160 = 1.5 : 1 (numerator not part of the denominator).
• Proportion female = 240 ÷ (240 + 160) = 240 ÷ 400 = 0.60 = 60% (numerator is part of the denominator).

Check: 160 × 1.5 = 240; 400 × 0.60 = 240.

Epidemiologic triangle (agent–host–environment). The classic model for infectious disease. Disease results from interaction among three elements; when they are in balance, disease does not occur (epidemiologic equilibrium).

Many versions add time (incubation period, duration of illness) to the center of the triangle.

Applying the triangle

• Malnutrition raising the risk of tuberculosis → host factor.
• Expanding mosquito breeding sites → environment factor.
• Vaccinating older adults against influenza → intervention on the host (raises immunity).
• Chlorinating drinking water to prevent typhoid → intervention on the environment.
• Building sidewalks and parks to prevent obesity → environment.

Limitation: the triangle fits infectious disease well but explains chronic diseases (heart disease, cancer) poorly, because they have many interacting causes and no single agent.

Web of causation. Proposed for chronic disease: many risk factors and indirect causes are linked in chains that interconnect like a web. Prevention can target any strand; breaking one link may reduce disease without removing every cause. Example: coronary heart disease linked to diet, inactivity, smoking, stress, genetics, hypertension, and dyslipidemia.

Wheel model. The host (with a genetic core) sits at the hub, surrounded by the biological, social, and physical environment. The size of each segment varies by disease (large genetic core for a single-gene disorder; large social environment for an injury). It de-emphasizes a single agent and suits chronic disease.

Social-ecological model. Health is shaped by nested levels: individual (knowledge, attitudes), interpersonal (family, peers), organizational (schools, workplaces), community (norms, built environment), and public policy (laws, regulations). Bronfenbrenner's ecological systems theory, on which it draws, uses the terms microsystem, mesosystem, exosystem, and macrosystem. Effective programs act at several levels at once.

Natural history of disease and levels of prevention (Leavell and Clark)

In Leavell and Clark's original five-level scheme (health promotion, specific protection, early diagnosis and prompt treatment, disability limitation, rehabilitation), disability limitation belongs to secondary prevention; many US texts instead place it under tertiary prevention. Follow the level the question's source uses; rehabilitation is always tertiary.

Some texts add primordial prevention (preventing risk factors from emerging, for example policies that make healthy food affordable) and quaternary prevention (protecting clients from overmedicalization).

Criteria for causation (Bradford Hill, 1965). An association is more likely to be causal when these are present:

1. Temporality — exposure precedes the outcome (the only essential criterion)
2. Strength of association (large relative risk)
3. Consistency across studies, places, and populations
4. Biological gradient (dose–response)
5. Plausibility (biologically sensible)
6. Coherence with known facts about the disease
7. Experiment (removing the exposure reduces disease)
8. Specificity (one exposure, one outcome — the weakest criterion)
9. Analogy with similar known cause–effect relationships

Smoking and lung cancer meet nearly all of these; this is the classic teaching example.

1. Describe the problem by person, place, and time using local data (vital records, surveillance reports, hospital discharge data, survey data such as BRFSS).
2. Choose a causation model that fits: triangle for an infectious outbreak; web of causation or social-ecological model for chronic disease or injury.
3. Map interventions to model elements. Example for childhood lead poisoning: agent (lead in old paint, pipes), host (young children with hand-to-mouth behavior, iron deficiency), environment (pre-1978 housing, poverty). Actions: remediate housing (environment), screen blood lead levels (secondary prevention), teach nutrition and hand washing (host).
4. Match level of prevention to stage. Immunization and education are primary; screening is secondary; rehabilitation is tertiary.
5. Use findings to set priorities and advocate for policy (for example, smoke-free laws target the environment level of the web).

• Data user and collector: gathers accurate data during home visits, clinics, and school health work; understands what the numbers mean.
• Case finder: identifies people at risk and refers them for screening and care.
• Educator: explains risk in plain language and teaches protective behaviors.
• Collaborator: works with epidemiologists, environmental health staff, and community partners.
• Advocate: uses epidemiologic evidence to push for policies and resources that address root causes and health inequities.
• Evaluator: measures whether interventions change disease occurrence.

• Population versus individual: epidemiology may justify actions that limit individual choice (for example, school vaccination requirements, isolation). States hold police power to protect public health; the US Supreme Court upheld state vaccination mandates in Jacobson v. Massachusetts (1905). Public health actions should use the least restrictive effective means.
• Privacy: the HIPAA Privacy Rule permits covered entities to disclose protected health information to a public health authority, without the client's authorization, for disease prevention and control. Share only the minimum necessary.
• Justice and equity: identifying high-risk groups must not stigmatize them; present data about communities respectfully and involve them in interpretation.
• Research ethics: analytic studies that go beyond routine public health practice need Institutional Review Board oversight and informed consent under the federal Common Rule.

Case 1 — Classifying factors. A nurse reviewing a tuberculosis cluster notes that most clients are malnourished, live in a crowded shelter, and have a drug-resistant strain.

Correct action: classify malnutrition as a host factor, crowded shelter as environment, drug resistance as an agent characteristic, and plan interventions for each (nutrition support, ventilation and screening at the shelter, directly observed therapy).

Why: the triangle organizes an infectious problem so no element is missed.

Case 2 — Choosing the right model. A county wants to reduce type 2 diabetes.

Correct action: use the web of causation or social-ecological model, not the triangle, and plan multilevel action (education, workplace wellness, safe walking routes, healthier school meals).

Why: chronic disease has multiple interacting causes and no single agent.

Case 3 — Level of prevention. A nurse organizes free blood pressure checks at a church.

Correct action: identify this as secondary prevention (screening for early detection) and arrange referral and follow-up for elevated readings.

Why: screening finds existing, unrecognized disease; it does not prevent onset.

• Calling screening "primary prevention." Screening is secondary; immunization is primary even though it is given in a clinic.
• Treating any association as causation. Temporality is required; cross-sectional data cannot show which came first.
• Using the total population as the denominator when only part of it is at risk.
• Confusing ratio and proportion: in a proportion the numerator is part of the denominator.
• Classifying vaccination as an environmental intervention — it changes the host.
• Assuming the triangle explains chronic disease well; its main limitation is multicausal chronic illness.
• Mixing up endemic (usual level) and epidemic (above expected level); an epidemic does not require large numbers — one case of a disease long absent can count as an outbreak.

• Epidemiology = distribution + determinants of health events in populations, applied to control problems.
• Descriptive = person, place, time (hypothesis generating); analytic = why (hypothesis testing).
• Triangle: agent, host, environment; balance = no disease; best for infectious disease.
• Web of causation and wheel model fit chronic disease; social-ecological model guides multilevel interventions.
• Vaccination targets the host; water treatment and vector control target the environment.
• Primary prevention = before disease (immunization, health promotion); secondary = early detection (screening); tertiary = limit disability (rehabilitation).
• Hill criteria: temporality is the only essential one; strength, consistency, and dose–response add weight.
• Ratio: numerator not in denominator; proportion: numerator in denominator; rate: includes time.
• HIPAA permits disclosure to public health authorities without authorization; Jacobson v. Massachusetts supports state police power.