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.

Image caption If the R-value is above one then the number of cumulative cases takes off, but if it is below one then eventually the outbreak stops. The further below one, the faster that happens.

Governments everywhere want to force the reproduction number down from about three to below one.

This is the reason you've not seen family, have had to work from home and the children have been off school. Stopping people coming into contact with each other to cut the virus's ability to spread.

What is the R number in the UK?

The reproduction number is not fixed. Instead, it changes as our behaviour changes or as immunity develops.

Mathematical modellers at Imperial College London are attempting to track how the number has changed as isolation, social distancing and the full lockdown were introduced.

Before any measures came in, the number was well above one and the conditions were ripe for a large outbreak. Successive restrictions brought it down, but it was not until full lockdown that it was driven below one.

It's a bit technical but by this point scientists refer to the Rt, rather than R, as the number is changing over time.

The figure now seems to be about 0.7. There is always some uncertainty in this type of work, but there is a high degree of confidence from the government's scientific advisers that the figure is below one overall.

The situation is different in some hospitals and care homes where the virus is spreading.

So how does this inform lifting lockdown?

As countries think about how to lift lockdown, the aim will be to keep the reproduction number below one.

Dr Adam Kucharski, from the London School of Hygiene and Tropical Medicine, told the BBC: "It's a big challenge making sure you're not loosening too much and increasing transmission."

However it has taken a monumental effort, one that has caused damage to people's lives, to get the number from three to 0.7.

"It doesn't give you a lot of room to play with [to keep the number below one]", Dr Kucharski added.

Earlier in April, Germany's reproduction number fell to about 0.7, the same as the UK's now.

But the Robert Koch Institute said that number had increased in recent days to one, before dropping back to 0.75.

"The number should stay below one, that is the big goal," said Prof Lothar Wieler, head of the institute.

Which measures could be lifted?

Unfortunately there is no confirmation how much each intervention affects the virus's spread, although there are estimates.

"Opening schools versus workplaces versus other gatherings - understanding how much they increase the reproduction number is going to be the challenge," said Dr Kucharski.

Another issue is that people's behaviour changes over time, so the number can creep up even if lockdown policies remain unchanged.

What is likely to be needed are new ways of controlling the virus, such as more extensive testing and tracing or location-tracking apps.

These can suppress the reproduction number in a more targeted way, allowing some of the other measures to be lifted.

Is it the most important number?

The reproduction number is one of the big three.

Another is severity - if you have a very mild disease that does not cause many problems then you can relax a bit. Coronavirus, and the disease it causes, Covid-19, is severe and deadly, unfortunately.

The last is the number of cases, which is important for deciding when to act. If you have a high number, but ease restrictions so the reproduction number is about one then you will continue to have a high number of cases.

What about a vaccine?

Having a vaccine is another way to bring down the reproduction number.

A coronavirus patient would naturally infect three others on average, but if a vaccine could protect two of them from infection then the reproduction number would fall from three to one.