Second, a bigger R0 doesn’t necessarily mean a worse disease. Seasonal flu has an R0 that hovers around 1.3, and yet it infects millions of people every year. SARS had an R0 of 2 to 5 and infected just over 8,000 people. The number is a measure of potential transmissibility. It does not actually tell you how fast a disease will spread.

“People make the mistake of thinking that a high R0 means that you’re inevitably going to end up with a pandemic, and that’s not what it means at all,” says Maia Majumder from Harvard Medical School, who published one of the seven estimates for the new virus. In her view, if the number is higher than 1, we should take the disease seriously. But exactly how high it is beyond that threshold isn’t very informative at this stage.

Why? Because third, R0 is an average. Let’s say the virus has an R0 of 2. This could mean that every single infected person passes the virus to two other people. It could also mean that one infected person is a “super-spreader” who infects 100 people, while 49 infected people infect no one. These two scenarios have radically different implications for what will happen during an outbreak.

Super-spreader events are dangerous for health-care workers, but counterintuitively, they can sometimes be a good sign.