There is a simple, but crucial number at the heart of understanding the threat posed by the coronavirus. It is guiding governments around the world on the actions needed to save lives, and it gives us clues to the extent that lockdown can be lifted. It is called the reproduction number or simply the R value. What is R? The reproduction number is a way of rating a disease’s ability to spread. It’s the number of people that one infected person will pass the virus on to, on average Measles has one of the highest numbers in town with a reproduction number of 15 in populations without immunity. It can cause explosive outbreaks. The new coronavirus, known officially as Sars-CoV-2, has a reproduction number of about three, but estimates vary. Why is a number above one dangerous? If the reproduction number is higher than one, then the number of cases increases exponentially – it snowballs like debt on an unpaid credit card. But if the number is lower, the disease will eventually peter out as not enough new people are being infected to sustain the outbreak.