That's interesting. I think I might have been at that course. Was it at Dhamma Kunja in July of 2012? If not, then pretty much the exact same thing happened on the exact day and time.
I remember being in deep meditation when I heard the laughter begin to spread. I thought, "hmm, I wonder if this will trigger one of those laughing fits I've heard about. Well, I'm not going to laugh, I'm meditating!"
Then about two seconds later it spread through the whole hall and I was laughing with them. Even after it died, it took all the concentration I had to not start again.
It has nothing to do with dhamma, but check out this radiolab
http://www.radiolab.org/story/91595-contagious-laughter/