Infectious Disease Prevention: Transmission Modes, Risk, and Protective Factors
Key Takeaways
- The chain of infection has six links: agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host; breaking any link prevents disease.
- Droplet transmission uses large particles that settle within about 6 feet (surgical mask); airborne uses small suspended particles requiring N95 respirators and negative-pressure rooms.
- Herd immunity threshold = 1 − 1/R0; for measles (R0 ≈ 12–18) the threshold is roughly 92–94%, for polio (R0 ≈ 7) about 86%.
- Risk factors include host susceptibility, behavioral exposure, environmental crowding and poor sanitation, and healthcare-associated invasive devices; protective factors include vaccination, hand hygiene, engineering controls, and policy mandates.
- Primary prevention prevents infection (vaccination, safe water), secondary detects and treats early (screening, contact tracing, post-exposure prophylaxis), and tertiary reduces complications and onward spread (ART for HIV, DOT for TB).
Quick Answer: Infectious disease prevention rests on breaking the chain of infection — agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host. CPH items test your ability to match each transmission mode (contact, droplet, airborne, vector-borne, common-vehicle) to its risk factors and the correct protective intervention, from vaccination and hand hygiene to ventilation and N95 respirators.
The Chain of Infection
Six links form the chain of infection. Breaking any one link prevents disease transmission:
| Link | Description | Example intervention |
|---|---|---|
| Agent | Microorganism with pathogenicity and virulence | Antimicrobial stewardship |
| Reservoir | Where the agent lives and survives (human, animal, environment) | Sanitation, deworming, water treatment |
| Portal of exit | Path out of reservoir (respiratory, blood, feces, skin) | Standard precautions |
| Mode of transmission | How the agent reaches a new host | Engineering and PPE controls |
| Portal of entry | Path into the new host (mucosa, skin break, inhalation) | PPE, hand hygiene |
| Susceptible host | Immunity, nutrition, comorbidities | Vaccination, treatment |
Transmission Modes and Their Distinctions
| Mode | Mechanism | Example diseases | Key control |
|---|---|---|---|
| Direct contact | Skin-to-skin or body-fluid contact | STIs, MRSA, scabies | Condoms, gloves |
| Indirect contact | Contaminated fomite | Norovirus, C. difficile, RSV | Hand hygiene, disinfection |
| Droplet | Large respiratory particles, short range (≤6 ft) | Influenza, pertussis, COVID-19 | Surgical mask, distancing |
| Airborne | Small particles (≤5 µm), suspended, long distance | TB, measles, varicella | N95, negative-pressure room |
| Common vehicle | Contaminated food, water, medication | Salmonella, cholera, hepatitis A | Food safety, water chlorination |
| Vector-borne | Mosquito, tick, flea | Malaria, dengue, Lyme, plague | Insecticide, bed nets |
| Nosocomial | Acquired in healthcare setting | CLABSI, VAP, CAUTI | Bundle protocols |
The droplet versus airborne distinction is the most-tested nuance. Droplets travel short distances and settle quickly, so surgical masks suffice. Airborne particles remain suspended and travel on air currents, requiring N95 respirators and negative-pressure isolation rooms. Measles is the classic airborne pathogen — one case can generate 12–18 secondary cases (R0) in a susceptible population.
Risk Factors for Infectious Disease
Risk factors increase exposure, susceptibility, or both:
- Host factors: immunocompromise, extremes of age, pregnancy, chronic disease, malnutrition, genetic susceptibility.
- Behavioral factors: unprotected sex, injection drug use, unsafe food handling, international travel, vaccine refusal.
- Environmental factors: household crowding, poor sanitation, inadequate ventilation, contaminated water, vector habitat.
- Healthcare factors: invasive devices (central lines, urinary catheters, ventilators), antimicrobial pressure driving resistance, lapses in sterilization.
Protective Factors
Protective factors reduce acquisition or severity of infection:
- Active immunity — vaccination, the most powerful primary prevention; herd immunity threshold depends on R0.
- Passive immunity — maternal antibodies and immunoglobulin (e.g., post-exposure prophylaxis for hepatitis A or B).
- Behavioral barriers — condoms, hand hygiene, safe injection practices, insecticide-treated bed nets.
- Engineering controls — ventilation, water chlorination, food refrigeration, sterile technique.
- Policy controls — school immunization mandates, quarantine, travel screening, reportable-disease surveillance.
Levels of Prevention Applied
- Primary — prevent infection: vaccination, safe water, condom promotion, vector control.
- Secondary — early detection and treatment: STI screening, contact tracing, tuberculin skin testing, post-exposure prophylaxis.
- Tertiary — reduce complications and prevent onward spread from known cases: antiretroviral therapy for HIV, directly observed therapy for TB, isolation of infectious TB cases.
Worked example: A county reports rising pertussis in schoolchildren. Primary intervention = Tdap vaccination and enforcement of school immunization mandates. Secondary = identify cases early via PCR and treat contacts with azithromycin. Tertiary = supportive care for infants, who bear the highest mortality. Because pertussis spreads by droplets, clinicians use surgical masks and cohorting — not N95 respirators, which are reserved for airborne pathogens.
Herd Immunity and the Reproductive Number
R0 is the average number of secondary cases produced by one infectious individual in a fully susceptible population. The herd-immunity threshold equals 1 − 1/R0. For measles (R0 ≈ 12–18), the threshold is about 92–94%; for polio (R0 ≈ 7), about 86%; for influenza (R0 ≈ 1.5), about 33%. Below the threshold, outbreaks propagate. Clusters of vaccine refusal can drop local coverage below the threshold even when national averages look adequate, producing the outbreaks CPH scenarios describe.
A related quantity, the effective reproductive number (Re or Rt), reflects transmission in a partially immune or partially distanced population. Re = R0 × susceptible fraction. When Re falls below 1, the outbreak declines; when Re exceeds 1, it grows. Non-pharmaceutical interventions (masking, distancing, ventilation, isolation) lower Re without changing R0, and exam items may ask which intervention most reduces Re rather than which reduces R0. A vaccination campaign that lifts population immunity above the herd threshold pushes Re below 1 and extinguishes sustained transmission.
Common Exam Traps
- Applying airborne precautions to a droplet pathogen (wasting N95s) or applying droplet precautions to measles (under-protecting staff and patients).
- Calculating herd immunity as 1/R0 instead of 1 − 1/R0.
- Classifying post-exposure prophylaxis as primary prevention — exposure already occurred, so it is secondary.
- Confusing reservoir with vector — a mosquito is a vector; standing water is the reservoir for the larval stage of the mosquito life cycle.
Which intervention is appropriate for an airborne pathogen such as measles in a healthcare setting?
For a disease with R0 = 8, what is the approximate herd immunity threshold?
Pertussis is transmitted by respiratory droplets. Which PPE is correct for clinicians evaluating pertussis cases?