Key Epidemiological Terms
Incidence: number of NEW cases in a population over a time period. Prevalence: total number of cases (new + existing) in a population at a point in time. Endemic: disease constantly present in a region. Epidemic: sudden increase above expected level. Pandemic: worldwide epidemic. Attack rate: proportion of exposed people who develop disease. Case fatality rate (CFR): deaths ÷ confirmed cases × 100%.
Rates & Ratios
Incidence rate = (New cases / Population at risk) × constant (1,000 or 100,000). Prevalence rate = (Total cases / Total population) × constant. Mortality rate = (Deaths / Population) × 100,000. Sensitivity: ability to correctly identify those WITH disease (avoid false negatives). Specificity: ability to correctly identify those WITHOUT disease (avoid false positives). PPV and NPV: depend on sensitivity, specificity, AND prevalence.
Screening Principles
Good screening test criteria: disease is important, has a detectable preclinical stage, and treatment in early stage improves outcomes. Test should be sensitive, acceptable to patients, and cost-effective. Screening ≠ diagnosis — positive screening requires confirmatory testing. Common screens: mammography (breast CA), Pap smear (cervical CA), colonoscopy (colorectal CA), FOBT, HbA1c (diabetes), lipid panel (CVD risk).
Chain of Infection
Six links: (1) Infectious agent (pathogen). (2) Reservoir (where pathogen lives). (3) Portal of exit (how it leaves reservoir). (4) Mode of transmission (contact, droplet, airborne, vehicle, vector). (5) Portal of entry (how it enters host). (6) Susceptible host. Breaking any link = prevention. Nursing interventions target every link: antibiotics (agent), isolation (exit/transmission), handwashing (entry), immunization (host susceptibility).
Outbreak Investigation
Steps: verify diagnosis, confirm outbreak, establish case definition, collect data (who/what/where/when), generate hypothesis, test hypothesis (epidemiological studies), implement control measures, report findings. Epidemiological triangle: Agent + Host + Environment. Community health nurse role: case identification, contact tracing, reporting to public health authorities, health education, coordination of response.
Important Notes
- Sensitivity is prioritized for screening tests (don't miss cases) — specificity for diagnostic tests (don't treat incorrectly).
- Mandatory reporting of notifiable diseases (TB, measles, HIV, hepatitis) to local public health department — this is a legal requirement.
Clinical Tips
- High sensitivity + low specificity = good screening test. High specificity + lower sensitivity = good confirmatory test.
- Prevalence affects PPV: low-prevalence diseases have high false-positive rates even with sensitive tests.
Summary
Epidemiology provides the quantitative tools to understand disease distribution and determinants in populations. Community health nurses apply epidemiological principles to prevent disease, detect outbreaks, and protect public health.
