R0
The average number of people one infected person will pass a disease to. An R0 above 1 means an epidemic grows; below 1, it fades. A simple number that captures how contagious a disease really is.
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Origin
The concept of a basic reproduction number originated in demography, but its modern epidemiological form was devised by George MacDonald in 1952 while studying malaria transmission. R0 became widely known during the COVID-19 pandemic. For reference: measles has an R0 of 12–18 (extremely contagious), the 1918 flu was 1.4–2.8, the common cold 2–3, and early COVID-19 estimates ranged from 2–6. An R0 below 1 means the disease will die out; above 1, it spreads.
Updated February 22, 2026