Key Formulas

Ratio · Proportion · Rate

• Ratio — one quantity compared to another; parts need not overlap (e.g., 3 boys : 2 girls)
• Proportion — part ÷ whole; numerator is included in the denominator; no time element (e.g., % of cases that are children)
• Rate — measures frequency of an event including a specific time period and population at risk (e.g., incidence rate)

Bias & Confounding

• Selection bias — how subjects are chosen/enrolled distorts the true exposure–disease relationship
• Recall bias — cases remember/report past exposures differently than controls (common in case-control studies)
• Information/measurement bias — inconsistent or inaccurate collection of exposure or outcome data
• Confounding — a third factor linked to both exposure & outcome distorts the apparent association
• Effect modification — a real (not spurious) difference in effect size across subgroups — not a bias to correct

History of Epidemiology

Surveillance Terms

• Sporadic — occasional, irregular cases
• Endemic — constant baseline level in an area
• Outbreak / Cluster — more cases than expected, localized
• Epidemic — outbreak spread over a wider area
• Pandemic — epidemic spread across countries/continents

Surveillance types:

• Passive — facilities routinely report cases
• Active — health dept. seeks out cases directly
• Sentinel — select sites report as a sample of the whole
• Syndromic — monitors symptom patterns before diagnosis

Epidemiologic Triad

• Agent — the pathogen/substance that must be present
• Host — the person/animal who can be infected (immunity, age, behavior)
• Environment — outside conditions bringing agent & host together (sanitation, crowding)

Chart Selection

Bar/histogram=epi curve, cases over time · Line graph=trend over time · Pie chart=proportions of a whole · Spot map=exact case locations · Area map=shades whole regions by rate · Scatter plot=correlation of two variables

CDC 13 Steps: Outbreak Investigation (write these)

1. Prepare for field workSciOly 1
2. Establish outbreak existsSciOly 2
3. Verify diagnosisSciOly 3
4. Construct case definitionSciOly 4
5. Find cases & record infoSciOly 4
6. Descriptive epi (time/place/person)SciOly 5
7. Develop hypothesesSciOly 6
8. Evaluate hypothesesSciOly 7
9. Refine & re-evaluate hypothesesSciOly 8
10. Compare w/ lab & environmentalSciOly 8
11. Implement control & preventionSciOly 9
12. Initiate/maintain surveillanceSciOly 9
13. Communicate findingsSciOly 10

SciOly steps: 1 Prepare · 2 Establish outbreak · 3 Verify diagnosis · 4 Define & identify cases · 5 Describe by time/place/person · 6 Develop hypotheses · 7 Evaluate hypotheses · 8 Refine hypotheses · 9 Control & surveillance · 10 Communicate findings

Study Designs

Descriptive (no comparison group):

• Case report/series — one or few detailed cases
• Cross-sectional — snapshot at one time; measures prevalence

Analytical (compares groups):

• Cohort (retro-/prospective) — groups by exposure, follow forward for disease. Yields RR
• Case-control — groups by disease status, look back for exposure. Yields OR. Controls: same source population & exposure opportunity as cases
• Ecological — compares whole groups/populations (not individuals); fast but risks the ecological fallacy

Experimental — researcher assigns exposure/treatment (e.g., RCT); strongest evidence for causation

Chain of Infection

Breaking any one link stops the chain — controls target the weakest link.

Portals of entry: mucous membranes, resp./GI tract, broken skin, parenteral. (Intact skin is a barrier, not a portal.)

Control by link: Agent—treat/disinfect · Reservoir—treat carriers · Portal of exit—cover, dispose safely · Transmission—sanitation, vector control · Portal of entry—PPE, safe prep · Host—vaccinate, prophylaxis

R₀ & Herd Immunity

Basic Reproduction Number (R₀) — average number of secondary cases caused by one case in a fully susceptible population.

• R₀ > 1 — disease spreads
• R₀ = 1 — stays stable (endemic)
• R₀ < 1 — dies out

Herd immunity threshold State/Nats = 1 − 1/R₀: share who must be immune to stop spread (measles R₀≈15 → 93%).

Causation Frameworks

• Koch's Postulates (4) — links a single agent to a disease
• Hill's Criteria (9): Strength, Consistency, Specificity, Temporality^*, Biological gradient, Plausibility, Coherence, Experiment, Analogy (^*essential one)
• Evans's Postulates — blends Koch+Hill · Causal Pie — component causes form a "pie"

Modes of Transmission

• Direct contact — touching, bodily fluids
• Indirect (fomite) — contaminated object
• Droplet — particles >5–10 µm; fall within a few feet
• Airborne — particles <5 µm; stay suspended, travel farther
• Vector-borne — mosquito, tick (mechanical or biological)
• Common vehicle — shared food, water, blood
• Fecal-oral — ingestion of fecal contamination

Don't confuse: Vector=living (mosquito) · Fomite=object (doorknob) · Vehicle=substance (water, food)

Vaccine & Immunity Types

• Active immunity — body makes its own antibodies (infection or vaccine); slower onset, long-lasting
• Passive immunity — ready-made antibodies (mother→infant = "maternal immunity"; antitoxin); immediate, temporary
• Live attenuated vaccine — weakened live pathogen; strong, long response (e.g., MMR)
• Inactivated vaccine — killed pathogen; safer, often needs boosters (e.g., IPV)
• Toxoid — inactivated toxin, not pathogen (tetanus) · Subunit — just a piece of the pathogen (Hep B)

Worked Example: AR & RR

Roles in an Investigation

Epidemiologist designs & analyzes; local/state health dept. is first responder; CDC assists on request (national surveillance/labs); laboratorian confirms pathogen; providers report & treat.

Line Listing & Lab Typing

Line listing — a spreadsheet with one row per case & columns for key variables (person/place/time, exposures, symptoms); the working tool for spotting patterns during an investigation.

PFGE / PulseNet — DNA fingerprinting that compares pathogen strains from different patients; matching patterns confirm cases share a common source (not just a similar illness).

Core Functions & Surveillance Steps

6 Core Functions of Epidemiology: Surveillance, Field investigation, Analytic studies, Evaluation, Linkages, Policy development.

5-Step Surveillance Process: Identify & define the problem → Collect data → Analyze & interpret → Disseminate findings → Evaluate & act (repeat).

Epidemic Curve Patterns

• Point-source — one sharp peak, rises fast, falls within ~1 incubation period. Everyone exposed at once.
• Continuous common-source — sustained plateau; drops only once source is fixed/removed.
• Propagated (person-to-person) — successive waves ~1 incubation period apart, often growing then shrinking.
• Intermittent common-source — irregular repeat peaks tied to on/off exposure (e.g., a well used sometimes).

Koch's Postulates

1. Agent found in all cases of the disease, absent from healthy hosts
2. Agent isolated from a diseased host & grown in pure culture
3. Cultured agent causes the disease when introduced to a healthy host
4. Agent re-isolated from that host, identical to the original

Doesn't fit every pathogen: viruses need host cells to culture, some carriers show no symptoms, and some diseases have multiple/unclear causes.

Natural History (CDC Stages)

Susceptibility → exposure → Subclinical disease (incubation/latency) → onset of symptoms → Clinical disease (usual time of diagnosis) → Recovery, disability or death

Vaccine Effectiveness State/Nats

VE = 1 − RR, RR = AR_vaccinated ÷ AR_unvaccinated. 5% vs 25% ill → RR 0.2 → VE 80% ("80% lower risk"). Best design: RCT vs placebo; concern: ethics.

Prevention & Disease Control

• Primordial — prevent risk factors from existing at all (e.g., laws limiting salt & trans fats in packaged food)
• Primary — before onset (vaccines, sanitation)
• Secondary — early detection & treatment
• Tertiary — reduce impact of existing disease
• Quaternary — avoid unnecessary/excessive care (e.g., antimicrobial stewardship)

4 stages of control: Control=reduced to acceptable level, needs upkeep (active outbreaks). Elimination=zero in one area, exists elsewhere (polio). Eradication=zero worldwide — only smallpox (1980) & rinderpest (2011). Extinction=agent gone from nature and labs (none yet).

Food Safety

Danger zone: 40°F–140°F — bacteria multiply fastest; max 2 hr (1 hr above 90°F). Cook poultry to 165°F, ground beef 160°F. Toxins (Staph, B. cereus) survive reheating.

What Makes a Disease Eradicable

Humans are the only reservoir (no animal/environmental source) · an effective, long-lasting vaccine or cure exists · clinical signs are easy to recognize (little silent/subclinical spread) · a reliable diagnostic test exists. Smallpox met all four; malaria and polio still don't (animal/environmental reservoirs or silent spread).

Reservoirs & Pathogen Types

Reservoirs: Human (case/carrier, e.g. typhoid) · Animal (zoonotic, e.g. rabies) · Environmental (soil/water, e.g. Legionella)

Agent types: Bacteria · Viruses · Fungi (e.g. ringworm) · Protozoa (e.g. malaria) · Prions (e.g. mad cow disease)

What Is Epidemiology?

The study of the distribution & determinants of health-related events in populations, and applying that study to control health problems.

Person, Place, Time

The 3 axes of descriptive epi — Person=who, Place=where, Time=when.

Case Definition

Built from clinical criteria (signs/symptoms), restrictions (person, place, time), and lab confirmation.

Confirmed = positive lab (even with no symptoms). Probable = typical symptoms, no lab yet. Possible/suspect = some symptoms, no lab. Not a case = lab shows a different agent, or criteria not met.

Why Investigate Outbreaks (5)

(1) Control/prevention · (2) Research opportunity · (3) Public, political, legal concerns · (4) Public health program considerations · (5) Training

Ecological Fallacy

Wrongly assuming a group-level (population) association applies to every individual within that group — a risk specific to ecological studies.

Lab Confirmation & Molecular Typing

PFGE / PulseNet — DNA fingerprinting comparing pathogen strains between patients; a match confirms a shared source. SNP mapping — compares tiny single-letter DNA differences between pathogen samples; higher resolution than PFGE for linking cases.

Quarantine vs. Isolation

Quarantine — restricts exposed, not-yet-sick people. Isolation — separates already confirmed/symptomatic people.

Infectious vs. Non-infectious

Infectious — caused by a transmissible pathogen (spreads person-to-person, vector, or vehicle). Non-infectious — caused by genetics, environment/toxins, or lifestyle (e.g., chemical exposure, diabetes) — cannot spread between people.

Clinical vs. Public Health Approach

Clinical — diagnoses & treats one patient at a time (e.g., a family doctor). Public health — prevents & controls health issues across whole populations (e.g., a health inspector, vaccination campaigns, policy). A school nurse screening every student for lice does both at once.

Landmark Studies

Framingham Heart Study (1948–) — cohort; found heart-disease risk factors (BP, cholesterol, smoking).
British Doctors' Study (Doll & Hill, 1951) — cohort; smoking causes lung cancer → anti-smoking policy.
Salk polio trials (1954) — placebo-controlled field trial, 1.8M kids; vaccine works → mass vaccination.
Nurses' Health Study (1976–) — diet/lifestyle & chronic disease in women.
Whitehall Studies (1967–) — lower job rank, higher death rates → social determinants.

Current Leaders (Fall 2026)

WHO Director-General: Tedros Adhanom Ghebreyesus. CDC Director: Erica Schwartz.