From yesterday's Guardian:
[my bold]
"A new study from the Office for National Statistics (ONS) suggested that one in 400 people had the virus outside hospitals and care homes
over the past two weeks, described by Prof Jonathan Van-Tam, the deputy chief medical officer for England, as “very low” circulation in the community."
So if I've understood correctly, that is (roughly) the number who are infectious in the community. So if i snog 400 random people in the street, I have a high chance of receiving a good dose. Just 50 and I will probably be OK