The Deceptively Simple Number Sparking Coronavirus Fears

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Updated February 4, 2020, at 10:35 a.m.

When a new disease emerges, health organizations turn to a seemingly simple number to gauge whether the outbreak will spread. It’s called the basic reproduction number—R0, pronounced R-—and though useful for decision makers, it’s a nightmare for public communication. In brief, R0 is the average number of people who will catch the disease from a single infected person, in a population that’s never seen the disease before. If R0 is 3, then every case will create three new cases. But even though it seems incredibly