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Epidemiology and Disease Control: Short Notes & Cheat Sheet

Measures of disease frequency, the chain of infection, levels of prevention, screening test performance, surveillance and eradication.

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Short notes

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Levels of prevention

from before disease to after itPrimordialStop risk factors appearingPrimaryPrevent disease: vaccines,educationSecondaryDetect early: screeningTertiaryLimit disability:rehabilitation
Earlier levels act before disease exists; later levels limit its damage.

Measuring disease

  • Incidence counts new cases in a population at risk over a period. Prevalence counts all existing cases at a point in time. Prevalence ≈ incidence × average duration.
  • Case fatality rate = deaths from a disease ÷ cases of that disease.
  • Infant mortality rate is per 1000 live births; maternal mortality ratio is per 100,000 live births.
  • Endemic: constantly present. Epidemic: more cases than expected. Pandemic: an epidemic across countries or continents. Sporadic: occasional, scattered cases.
  • Herd immunity threshold = 1 − 1/R₀. For measles (R₀ about 12–18) this is about 92–95%.

Prevention and screening

  • Primordial prevention stops risk factors from arising; primary prevention stops disease (vaccination, health education); secondary prevention detects it early (screening); tertiary prevention limits disability (rehabilitation).
  • Sensitivity is the proportion of people with the disease who test positive; specificity is the proportion without it who test negative.
  • A highly sensitive test, when negative, rules disease out (SnNout). A highly specific test, when positive, rules it in (SpPin).
  • Predictive values depend on prevalence: the rarer the disease, the lower the positive predictive value.

Control

  • Chain of infection: agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host. Breaking any link stops spread.
  • Isolation separates sick people; quarantine restricts healthy exposed people for the longest incubation period.
  • Surveillance can be passive (routine reports), active (searching for cases) or sentinel (selected sites).
  • Control reduces disease; elimination removes it from a region; eradication removes it worldwide. Smallpox was eradicated in 1980.
  • Wild poliovirus is still endemic only in Pakistan and Afghanistan.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Formulas

MeasureFormula
SensitivityTP ÷ (TP + FN)
SpecificityTN ÷ (TN + FP)
Positive predictive valueTP ÷ (TP + FP)
Negative predictive valueTN ÷ (TN + FN)
Prevalence≈ incidence × duration
Herd immunity threshold1 − 1/R₀

Levels of prevention

LevelExample
PrimordialHealthy school food policies
PrimaryVaccination, iodised salt
SecondaryBP screening, Pap smear
TertiaryStroke rehabilitation

Memory aids

SnNout and SpPin
Sensitive test Negative rules OUT; Specific test Positive rules IN.

Exam traps

  • Prevalence, not incidence, is what a cross-sectional survey measures.
  • Screening is secondary prevention, not primary.
  • Positive predictive value falls when prevalence falls, even if the test itself is unchanged.
  • Quarantine applies to healthy exposed people; isolation applies to the sick.

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