It's the first day of new restrictions on movement in the UK. So what does the British government hope to achieve?
Scientists currently believe that each person infected with Covid-19 infects, on average, a further 2.5 people over the course of around five days.
That means 30 days after that first infection, 406 people will have the virus.
But, if we reduce our exposure by half, that first infection would only lead to 15 infections in 30 days - less than 5% of 406.
Of course, it’s not guaranteed that we can all reduce our exposure by half and it takes a few weeks for the effect of any intervention to show up.
But if we reduce exposure by, say, a third, the reduction is just over 85% (or 53 infections).
Look at the diagram below, which may make the effect of reducing the infection rate a little clearer.
First published by Robert Cuffe, BBC head of statistics. Click here