For what its worth, I caught chicken pox twice. I caught it bad the first time and not so bad the next time. Delightful!
As far as strangles goes, it scares the crap out of me. We had a friend almost lose her horse to it. Under your same circumstances, she kept boarding at a place where they let people come in as they please and never seperated anyone. Hell, they even had shows at their barn. Her horse got '******* strangles' and barely made it back. He has made a full recovery but boy was it tough and expensive.
Strangles is HUGE in my area. We have several barns that no one will even go look at another horse at because they are *known* for being a 'strangles' barn. Its some scary, hiiiiiighly contagious stuff.