And now it seems to be distance and time.
Always was, but that is a difficult message to get over to the general public without misunderstandings, so the time element got dropped. With a phone app to do the monitoring, time can be brought into the equation.
It's a question of viral load: just one particle has very little chance of infecting a healthy person, it takes more of an onslaught.
It is also my opinion (rather than specific knowledge) that coronavirus can only take hold if inhaled, or to contract it from a contaminated surface would require considerable transport into the mouth (or other thin membrane rather than skin). It is unlikely to be contracted through the stomach, where the acids should deal with it.
However: if you have a compromised immune system, all bets are off. Just one
might get through.
So:
Normal heathy people, even elderly people,
can go about their business by being cautious about being around other people (outside their household), ie social distancing, and by being careful about decontaminating themselves if they come into contact with anything - in other words: don't touch anything if you can avoid it, don't eat or drink out of the house, and wash your hands the moment you get home.
Immune-compromised people need to avoid
any contact, which is the purpose of self-isolation - but that has been advised for
all the over-70's instead of just the immune-compromised group, which has weakened the message.
However, in my view, enclosed spaces such as supermarkets are a risk despite social distancing, even to non-immune-compromised people. Some people have to do the shopping, so then you have to consider risk of serious illness or death as opposed to risk of any illness. Over-70's and people with underlying conditions (but not immune-compromised) shouldn't need to self-isolate, but neither should they do the shopping.
BUT: the big elephant in the room is how you get all those ifs-and-buts over to the general public. If there is any chance of confusion, you have to simplify... and that's what we've got. And don't take my word for it either - if you decide not to self-isolate and catch it, I'm not accepting blame!