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.
What a learner can do afterwards
- 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.
1 · Read
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.
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.
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.
Incidence tracks speed, prevalence tracks burden, and immunity holds the reproduction number below one.
More to read
16.1 The Language of Epidemiologists
Epidemiologists split counting into two questions. Incidence counts new cases over a period and shows how fast disease is spreading right now. Prevalence counts all current cases and shows the total burden sitting in the population. The two can move in opposite directions: a long lasting mild illness piles up high prevalence while incidence falls. Confusing them misreads the outbreak, celebrating a calmer spread while clinics stay full.
16.2 Tracking Infectious Diseases
Tracking turns case counts into decisions. Laboratories confirm what is circulating, clinicians report who is sick, and public health teams map where and when cases cluster. The reproduction number summarises the trend: above one means each case breeds more than one successor and the outbreak grows, below one means it shrinks. Watching that number cross one matters more than any single day count. Surveillance exists to catch the crossing early.
16.3 Modes of Disease Transmission
Transmission mode decides which interventions bite. Respiratory spread answers to ventilation and masks, waterborne spread to clean water, vector borne to controlling insects, and contact spread to hygiene and isolation. Each mode implies a different reproduction number because contact rates and infectious periods differ. Cutting the effective number below one is the universal goal; the route there depends on the mode.
34.3 Epidemics and Pandemics
Epidemics and pandemics differ in scale, not in logic. Both grow while the effective reproduction number sits above one and fade when protection or depleted susceptibles push it below. The immune fraction needed follows the simple rule that each case must meet too few susceptibles to replace itself. Protection can come from recovery or vaccination; the arithmetic does not care which. Hold the effective number down and the curve bends whatever the pathogen.
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2 · Watch
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Where it sits
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Nothing; this is a starting point.
Optional connections
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.