Disease Prevention and Control: Transmission, Immunity, and Public Health Measures
Core concepts
Distinguish communicable (infectious) from non-communicable diseases such as diabetes, cardiovascular disease, and most cancers. For infectious disease, know the chain of infection: pathogen, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host. Breaking any link interrupts transmission.
Know transmission modes: direct contact, droplet (large respiratory particles, short range), airborne (small particles that travel farther, e.g., tuberculosis, measles), vector-borne (mosquitoes, ticks), vehicle-borne (food, water, blood), and fomite contact. Standard precautions (hand hygiene, gloves, safe sharps handling) apply to all patients; transmission-based precautions add measures for specific pathogens.
Understand immunity: innate (nonspecific barriers and inflammation) versus adaptive (specific, with memory). Active immunity occurs when the body makes its own antibodies after infection or vaccination; passive immunity is borrowed antibody (maternal transfer through placenta or breast milk, or immunoglobulin therapy) and is temporary. Herd immunity protects those who cannot be vaccinated when enough of the population is immune.
Know the public health vocabulary: epidemic (above-normal cases in a region), pandemic (global spread), endemic (habitual presence), incidence (new cases) versus prevalence (all existing cases).
Traps
Questions swap active and passive immunity, or label a foodborne illness as airborne. Another trap confuses incidence with prevalence. Watch for "all" or "always" statements about precautions.
Worked example / rule of thumb
Rule of thumb: did the person's body do the work? Yes → active immunity (vaccine, natural infection). No, antibody came from outside → passive (breast milk, immunoglobulin). Example: an infant receiving antibodies through breast milk has passive immunity; a child immunized against measles develops active immunity.
Exam strategy
Identify the link in the chain being broken (e.g., handwashing breaks portal of exit/entry) before selecting an answer.