Thanks everyone for the warm welcome and really helpful information.

It's a bit tricky for me to give specific details about how I was exposed; if I put specific details on a web forum I've realised it would be very easy for people who know some of the detail of what happened to work out exactly who I am, and I'd like to preserve my anonymity if possible. It was a really unusual situation and it happened while I was at work. Sorry to sound so vague!