--- title: "Epidemiology: Incidence, the Reproduction Number and Tracking an Outbreak" description: "How fast a disease spreads is summarised by the average number of new cases one case produces, and that number falls as susceptible people run out or are protected. Incidence and prevalence answer dif" canonical: https://lightmysky.com/learn/science/epidemiology-incidence-the-reproduction-number-and-tracking-an-outbreak-mt_f2tT08sEki source: https://lightmysky.com/learn/science/epidemiology-incidence-the-reproduction-number-and-tracking-an-outbreak-mt_f2tT08sEki.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Epidemiology: Incidence, the Reproduction Number and Tracking an Outbreak How fast a disease spreads is summarised by the average number of new cases one case produces, and that number falls as susceptible people run out or are protected. Incidence and prevalence answer different questions, and confusing them misreads an outbreak. Subject: Science · Area: Microbiology · Ages 21 to 22 Page: https://lightmysky.com/learn/science/epidemiology-incidence-the-reproduction-number-and-tracking-an-outbreak-mt_f2tT08sEki ## Ready when they can - Distinguishes incidence from prevalence with a case where they move in opposite directions. - Explains what a reproduction number above and below one implies for an outbreak. - Calculates the proportion that must be immune to hold the effective number below one. ## Lesson: Counting cases and stopping spread Epidemiologists ask two different counting questions. Incidence counts new cases over a period, showing how fast disease spreads right now. Prevalence counts all current cases, showing the total burden sitting in the population. In 2012, new HIV diagnoses in the US gave an incidence of 13.7 per 100,000, while all people living with HIV gave a prevalence of 467 per 100,000. **Example.** Picture a long mild illness entering a town. New cases per week fall, so incidence drops and leaders celebrate. But patients stay sick for months, so the total sick pile grows and clinics stay full. The two counts move in opposite directions, and confusing them misreads the outbreak. Tracking turns counts into decisions. Labs confirm what circulates, clinicians report who is sick, and teams map where cases cluster, the way John Snow mapped cholera cases to the Broad Street pump. The reproduction number sums up the trend: above one, each case breeds more than one successor and the outbreak grows, and below one, chains die out and it shrinks. **Tip.** To hold spread down, multiply the starting number by the susceptible share and keep the result below one. If each case would cause 4 next, keep susceptibles below 1 in 4, so more than 3 in 4 must be immune. Shrink the susceptible share through immunity until each case replaces itself less than once. **Recap.** Incidence tracks speed, prevalence tracks burden, and immunity holds the reproduction number below one. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [How Communicable Diseases Spread](https://lightmysky.com/learn/science/how-communicable-diseases-spread-mt_o__-bF814q) ## Opens up - [Clinical Trials: Phases, Endpoints and Why Most Candidates Fail](https://lightmysky.com/learn/science/clinical-trials-phases-endpoints-and-why-most-candidates-fail-mt_xKBKzSYxLh)