Epidemiology: Incidence, the Reproduction Number and Tracking an Outbreak · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Counting cases and stopping spread

Science · Microbiology · ages 21-22
Name ______________________   Date ____________

  1. The reproduction number of an outbreak is 0.7. What happens?

    • The outbreak grows
    • The outbreak shrinks
    • Every case causes exactly one next

  2. A town reports 40 new cases this week and 500 people currently sick. Which is the incidence?

    • The 40 new cases this week
    • The 500 currently sick
    • The town population

  3. Prevalence counts all current cases, while incidence counts new cases over a period.

    Circle one:   True   False

  4. Why did mapping cholera cases to one pump matter more than any single day count?

    • It found the shared source to remove
    • It proved water is always safe
    • It counted the whole city at once

  5. New cases fall but the sick total keeps climbing. How is that possible?

    • The population suddenly doubles
    • Recovered patients are counted twice
    • The illness lasts long, so cases pile up

  6. Each case would cause 5 next. What share of the population must be immune to stop growth? Type the decimal.

    Answer: ______________

  7. Each case would cause 4 next, and 1 in 2 people are immune. Does the outbreak grow?

    • No, because half immune always stops spread
    • No, because 4 is below the danger line
    • Yes, because 4 times 1 in 2 is still 2

  8. A student says a reproduction number of exactly 1 means the outbreak is over. What is wrong?

    • At 1 the disease turns milder
    • At 1 each case still replaces itself, so spread holds steady
    • At 1 prevalence must be zero
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Counting cases and stopping spread W1-mt_f2tT08sEki-s1

  1. The outbreak shrinks · Below one, chains of spread die out faster than they branch.
  2. The 40 new cases this week · Incidence is the flow of new cases over a period, not the standing total.
  3. True · One is the standing burden, the other is the speed of arrival.
  4. It found the shared source to remove · Clustering cases around the pump fingered the common source, and removing the handle contained spread.
  5. The illness lasts long, so cases pile up · A long illness banks prevalence faster than falling incidence drains it.
  6. 0.8 · Keep susceptibles below 1 in 5, so more than 4 in 5 must be immune.
  7. Yes, because 4 times 1 in 2 is still 2 · Half susceptible times 4 gives 2 successors each, still above one, so growth continues.
  8. At 1 each case still replaces itself, so spread holds steady · One means flat, not gone: stopping growth needs the number held below one.
Worksheet · LightMySky