Presumably either not well ventilated, or virus on surfaces touched by all. Sad news.
It will be assumed that the tradesman had it first, and brought it with him, as he is presumably visiting many people. But could it have gone the other way? They had it first?
We see a lot of data about the spread and its effects. We are encouraged to wear masks and cleanse hands regularly, but do we have any details of exactly how some have been infected? Difficult to gain, I imagine.
A friend of mine, their daughter (mid twenties) got it, isolated in one room, but of course had to share loo and bathroom. She has recovered. Then my friend got it, isolated, got worse, just come home, thankfully, from a week in hospital. His wife appears to have escaped. So something is working for them, suggesting the basic rules we are given can work.