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-naught—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 on average every case will create three new cases. But even though it seems incredibly straightforward, it’s hard to calculate and tricky to interpret.
R0 is important because if it’s greater than 1, the infection will probably keep spreading, and if it’s less than 1, the outbreak will likely peter out. So it offers vital information to organizations and nations as they consider how to respond to an outbreak—such as the one the world is currently experiencing.
In December, a previously unknown coronavirus, now called 2019-nCoV, emerged in the Chinese city of Wuhan. There have been more than 4,500 confirmed cases, the majority of which have been in mainland China. But several dozen cases have been detected in more than 15 other countries, and as the outbreak has spread, so has fear. Public-health researchers have sped to estimate the R0 of the new disease, and as they have shared their findings, this number has fueled several alarmed missives on social media.