DS was 14m when I got a message warning me that the child he'd played with a couple of days earlier had broken out with chicken pox. The mum hadn't knowingly exposed either child to it, it's just one of those things.
I started a new job 2 days later. In the end, he didn't catch it on that occasion, and DH and I certainly couldn't have justified 2 weeks off work just in case DS was going to be ill.
After several exposures at nursery, he finally got it soon after the birth of DS2. Because I was still struggling and recovering, we hadn't been out much in the contagious phase. Most of the time that DS2 was potentially contagious we were in because of DS1. Naturally DS2 broke out with it towards the end of the contagious period having reached a strapping 9 weeks old.
Being exposed to the illness isn't a guarantee of catching it, so we can't go off into quarantine just in case as a matter of course. In families with siblings, that could easily add up to a month or more!
It is fair to warn people and let them come to their own conclusions.
As DS2 was just about crusting over, we had a Brownie off site activity. Another leader was pregnant and struggling with the nature of the activity, so she missed it. I had to go for adult ratios, so took DS2 in the pram with rain covers on and kept to the rear of the group to minimise the risk of contact with anyone else.
Society also relies on the fact that most people only get it once, although this is not foolproof.