3. Natural History, Chain of Infection, and Levels of Prevention
After this lesson you can
- Place exposure, the detection threshold, symptom onset, diagnosis and outcome on the natural history timeline
- Distinguish the incubation period from the latent period and name the induction period
- Define infectivity, pathogenicity and virulence by their denominators
- Name the six links of the chain of infection and match a control measure to each
- Classify an intervention as primordial, primary, secondary or tertiary prevention
- Compute a herd immunity threshold and the vaccine coverage needed when the vaccine is imperfect
A lesson on describing a disease in one person over time. It covers the natural history timeline and its periods, the spectrum of disease, the epidemiologic triad, the six-link chain of infection, and the four levels of prevention. It closes with reproduction numbers and the herd immunity threshold, worked on Oakhaven's fictional respiratory agent.
Questions and answers
Questions a learner would ask about this lesson. Think through your own answer before you open each one.
- How can someone be infectious before their first symptom?
Show answer
The infectious latent period runs from exposure to the onset of infectiousness, while the incubation period runs from exposure to the onset of symptoms. Both start at exposure but end at different events, and they can end in either order. When the latent period is shorter than the incubation period, spread happens before symptoms appear. For Oakhaven's respiratory agent the latent period is about two days shorter.
- What are infectivity, pathogenicity and virulence?
Show answer
They are three proportions along the spectrum of disease, each with a different denominator. Infectivity is infected over exposed. Pathogenicity is clinically apparent over infected. Virulence is severe or fatal over clinically ill. Each denominator is the previous numerator.
- What are the six links in the chain of infection?
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The links are the agent, the reservoir, the portal of exit, the mode of transmission, the portal of entry and the susceptible host. Breaking any one link stops transmission, and each link has a matching control, such as isolating cases at the reservoir or vaccination for the susceptible host. The new host becomes the next reservoir.
- How do I tell primordial, primary, secondary and tertiary prevention apart?
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Primordial prevention stops risk factors from emerging in a population, for example through policy or taxation. Primary prevention stops disease in a well person who is at risk, for example vaccination or protective equipment. Secondary prevention detects disease early and treats it promptly, and screening belongs here. Tertiary prevention limits disability and restores function once disease is established.
- Why does vaccine coverage need to be higher than the herd immunity threshold?
Show answer
The threshold counts immune people, not vaccinated people. With an R naught of 4, the threshold is 1 minus one quarter, or 75 percent. With a vaccine that is 80 percent effective, coverage needed is 75 divided by 0.8, or about 94 percent.
Practice quiz
- Question 1The interval from exposure to the onset of infectiousness is the:
- Question 2Among 100 infected people, 10 develop symptoms. What is the pathogenicity?
- Question 3A test that finds disease in people without symptoms is which level of prevention?
- Question 4Droplet spread within a few feet is classified in this course as:
- Question 5For an R naught of 4, the herd immunity threshold is:
- Question 6Oakhaven's agent has an R naught of 4, and half the district is susceptible. What is the effective reproduction number, R t?
- Question 7Among 50 susceptible household contacts of a case, 10 fall ill. What is the secondary attack rate?
- Question 8A tax that keeps a risk factor from taking hold across a population is which level of prevention?
- Question 9The detectable preclinical phase runs from:
- Question 10What is the difference between a mechanical vector and a biologic vector?
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Principles of Epidemiology is education in epidemiologic reasoning, not clinical, screening, or policy guidance. Every example is set in the fictional Oakhaven Health District, and every number is invented for teaching.