Chain of infection. A communicable disease spreads when six links connect: infectious agent → reservoir → portal of exit → mode of transmission → portal of entry → susceptible host. Control means breaking at least one link.
| Link | Examples | Control measures |
|---|
| Agent | Bacteria, viruses, fungi, parasites, prions | Early diagnosis and treatment, antimicrobials |
| Reservoir | Humans (cases, carriers), animals (zoonoses), soil, water | Treat cases and carriers, animal control, sanitation |
| Portal of exit | Respiratory secretions, feces, blood, skin lesions | Respiratory etiquette, covering lesions, safe waste disposal |
| Mode of transmission | Contact (direct/indirect), droplet, airborne, vehicle (food, water), vector | Hand hygiene, isolation precautions, safe food and water, vector control |
| Portal of entry | Mucous membranes, broken skin, respiratory or GI tract | PPE, wound care, safe injection practices |
| Susceptible host | Unvaccinated, immunocompromised, very young or old | Immunization, prophylaxis, nutrition |
Three classic strategies of control: (1) eliminate or treat the source (reservoir), (2) interrupt transmission, (3) protect the susceptible host. When a specific contaminated product is identified (for example, oysters in a hepatitis A outbreak), removing it from sale is the most direct source-control action.
Time periods
- Incubation period: exposure to first symptoms.
- Latent period: infection to onset of infectiousness.
- Period of communicability: time during which the agent can be transmitted.
- Carrier: infected person without symptoms who can transmit the agent.
Isolation vs. quarantine
- Isolation separates ill (infected) people from others.
- Quarantine separates and restricts movement of exposed people who are not yet ill for the incubation period.
Occurrence terms: sporadic, endemic, outbreak/epidemic (more cases than expected in a place and time), cluster (cases grouped in time and place, cause not yet established), pandemic.
Herd (community) immunity. When enough people are immune, transmission chains break and unvaccinated people are indirectly protected. Threshold ≈ 1 − 1 ÷ R0, where R0 (basic reproduction number) is the average number of new infections caused by one case in a fully susceptible population.
Levels of prevention in communicable disease
- Primary: immunization, pre-exposure prophylaxis, safe water and food, condom distribution, vector control, education.
- Secondary: screening and early diagnosis (latent TB testing, HIV and STI testing), contact tracing, prompt treatment. Post-exposure prophylaxis (for example, hepatitis A vaccine or immune globulin, rabies PEP) is classified as primary prevention in many texts because it prevents disease from developing after exposure; some texts list it with secondary measures.
- Tertiary: preventing complications and disability in people already ill (rehabilitation after polio or meningitis, managing chronic hepatitis).
Public health surveillance is the ongoing, systematic collection, analysis, interpretation, and dissemination of health data for action.
| Type | How it works | Example |
|---|
| Passive | Providers and labs report as required; the health department waits for reports | Routine notifiable disease reporting — inexpensive but prone to underreporting |
| Active | The health department contacts providers, labs, or facilities to seek cases | During an outbreak |
| Sentinel | Selected sites report on a sample | Outpatient influenza-like illness networks |
| Syndromic | Near-real-time data on symptoms before diagnosis | CDC National Syndromic Surveillance Program (NSSP), which uses emergency department visit data through the BioSense Platform |
US notifiable disease system
- State and local laws specify which diseases health care providers, laboratories, and others must report to the state or local health department, and in what time frame. Some conditions (for example, suspected bioterrorism agents) require immediate telephone reporting.
- States and territories then voluntarily notify CDC through the National Notifiable Diseases Surveillance System (NNDSS).
- The Council of State and Territorial Epidemiologists (CSTE), with CDC, recommends which conditions are nationally notifiable and sets standard surveillance case definitions (commonly classified as confirmed, probable, or suspected).
- A nurse reports to the local or state health department, not directly to CDC.
Epidemic curves
| Pattern | Curve | Example |
|---|
| Point source | Sharp rise and fall within one incubation period | Contaminated potato salad at a picnic |
| Continuous common source | Rise then plateau while exposure continues | Contaminated municipal water |
| Propagated (person-to-person) | Successive, progressively taller peaks about one incubation period apart | Measles in an undervaccinated school |
Outbreak investigation (CDC Field Epidemiology Manual, 10 steps) — steps may overlap or run at the same time:
- Prepare for field work
- Confirm the diagnosis
- Determine the existence of an outbreak (compare with expected numbers)
- Identify and count cases (create a case definition — person, place, time, clinical criteria)
- Tabulate and orient the data by time, place, and person (epidemic curve, spot map)
- Consider whether control measures can be implemented now
- Develop and test hypotheses
- Plan one or more systematic studies (cohort or case-control)
- Implement and evaluate control and prevention measures
- Communicate findings
Each calculation below was checked twice.
Example 1 — Food-specific attack rates. Sixty people attend a lunch; 32 become ill with vomiting within 6 hours.
| Food | Ate: ill / total | Attack rate | Did not eat: ill / total | Attack rate | RR |
|---|
| Chicken salad | 30 / 40 | 75% | 2 / 20 | 10% | 7.5 |
| Potato chips | 16 / 30 | 53.3% | 16 / 30 | 53.3% | 1.0 |
- Overall attack rate = 32 ÷ 60 × 100 = 53.3%
- Chicken salad: 30 ÷ 40 = 0.75; 2 ÷ 20 = 0.10; RR = 0.75 ÷ 0.10 = 7.5
- Potato chips: 16 ÷ 30 = 0.533 in both groups; RR = 1.0
Conclusion: chicken salad is the likely vehicle — high attack rate among those who ate it, low among those who did not, and most ill people ate it.
Example 2 — Herd immunity threshold. Measles R0 is often cited as 12–18. Using R0 = 15:
Threshold = 1 − 1 ÷ 15 = 1 − 0.067 = 0.933 ≈ 93% of the population must be immune. With R0 = 12: 1 − 0.083 = 0.917 ≈ 92%; with R0 = 18: 1 − 0.056 = 0.944 ≈ 94%.
Example 3 — Secondary attack rate. A child with measles lives in a household with 6 contacts; 2 have documented immunity (2 doses of MMR vaccine). Two of the remaining contacts become ill.
Susceptible contacts = 6 − 2 = 4. Secondary attack rate = 2 ÷ 4 × 100 = 50%.
Contact investigation (for example, pulmonary TB)
- Interview the client about the infectious period and places spent.
- List close contacts and prioritize by exposure (household, infants, immunocompromised).
- Test contacts (TB blood test or skin test, symptom review, chest X-ray if positive).
- Refer for treatment of latent infection or disease; provide directly observed therapy (DOT) or video DOT for active TB to ensure adherence.
- Maintain confidentiality — do not reveal the index client's identity to contacts.
- Recognize and report notifiable conditions promptly to the health department according to state law.
- Case investigation and contact tracing: interview cases, identify contacts, arrange testing, prophylaxis, or quarantine.
- Immunization: assess records, catch up missed doses, counter misinformation with clear, respectful teaching; follow the current CDC schedules and state requirements.
- Treatment adherence: DOT for TB, partner services for STIs and HIV, follow-up of positive tests.
- Infection control in community settings: schools, shelters, long-term care, child care.
- Health education and risk communication during outbreaks: consistent, accurate, culturally and linguistically appropriate messages.
- Surveillance support: accurate data, line lists, and documentation.
- Reporting is mandatory under state law; the HIPAA Privacy Rule permits disclosure to public health authorities without client authorization.
- Isolation and quarantine are mainly state and local powers (police power). The federal government, through CDC under the Public Health Service Act, can act to prevent spread of specified diseases across state lines and into the country. Orders must follow due process and use the least restrictive means.
- Vaccination requirements for school entry are set by each state, including which exemptions are allowed; the courts have long upheld state authority (Jacobson v. Massachusetts, 1905).
- Confidentiality in contact tracing: protect the identity of the index case.
- Ethics: balance individual liberty with community protection (utilitarian vs. individual rights); avoid stigmatizing groups linked to an outbreak; ensure equitable access to vaccines and treatment.
Case 1 — Reporting. A school nurse suspects measles in a student with fever, cough, conjunctivitis, and a spreading rash.
Correct action: isolate the student from others (airborne precautions), notify the local health department immediately, check immunization records of contacts, and follow health department guidance on exclusion of susceptible students.
Why: measles is highly contagious; rapid reporting enables post-exposure prophylaxis and exclusion.
Case 2 — Isolation or quarantine? A traveler was exposed to a disease with a 10-day incubation period but is well.
Correct action: quarantine (monitoring and movement restriction) if public health orders it, not isolation.
Why: isolation is for ill persons; quarantine is for exposed, not-yet-ill persons.
Case 3 — Source control. Hepatitis A cases are traced to raw oysters from one distributor.
Correct action: support removal and recall of the product, notify people who ate it, and arrange post-exposure prophylaxis (vaccine or immune globulin as indicated) within 2 weeks of exposure.
Why: eliminating the source is the most direct control measure.
- Reporting directly to CDC instead of the local or state health department.
- Confusing isolation (ill) with quarantine (exposed, well).
- Waiting for laboratory confirmation before reporting a condition that requires immediate notification of suspected cases.
- Using all household members, including immune ones, as the secondary attack rate denominator.
- Calling vaccination secondary prevention (it is primary) or contact tracing primary prevention (it is secondary).
- Misreading a propagated curve (multiple peaks) as a point-source outbreak.
- Implementing control measures only after the investigation is finished — control should start as soon as the likely source is known.
- Believing passive surveillance captures all cases; underreporting is common.
- Chain of infection: agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host.
- Control: remove source, break transmission, protect host.
- Isolation = ill; quarantine = exposed but well.
- Herd immunity threshold ≈ 1 − 1 ÷ R0 (measles about 92–94%).
- Passive (routine reports), active (health department seeks cases), sentinel (sample sites), syndromic (NSSP, emergency department data).
- Providers report to the state or local health department by state law; states voluntarily notify CDC via NNDSS; CSTE and CDC set the national list and case definitions.
- Point source: one sharp peak; propagated: successive peaks one incubation period apart.
- Outbreak investigation starts with confirming the diagnosis and the existence of an outbreak, then a case definition; control measures may begin early.
- Food-specific attack rates and RR identify the likely vehicle.