A colleague of mine is isolating because his daughter tested positive last Sunday, last I heard he hadn't been tested and had no symptoms, but I'm trying to stay out of the loop for now, as I was on the desk the day after he was last there. Luckily I'm paranoid about cleanliness since early this year, but I worked that day with 3/4 others who'd been with him for the 2 previous days, and this exactly shows how it can be spread.
Kid I've never met, and live 30 miles from it, caught it, probably at school, she might have given it to her dad (we don't know yet but given he's in the house with her quite likely), who might then have given it to 3/4 colleagues he worked with for 12 hours, twice, who subsequently could have given it to me, all over several days.
Yes lots of "might be's" obviously, but people need to know/think how it can spread whilst everyone involved is still being sensible, and following the rules, and all of this in tier 4.