Hi, Susanne
As I’m studying the English version of your “Letters From My Cottage In England”, I noticed a mistake in Letter#1. Indeed, you wrote, “Ce rire infectieux”, and at the moment I corrected it, I couldn’t find a satisfying expression since I didn’t really get what you meant.
But, to the light of the english version, things are now clearer. The French translation that fits “Infectious laugh” is “rire communicatif”.
It’s the way that people laugh that makes you laugh, not the joke they are telling you. You are infected by the laughter the same way you would be infected by the flu. Hearing a baby laugh makes you laugh but I doubt the baby’s joke was hilarious 
I love it when I learn something new. I shall attempt the technical challenge to correct this both here and on Posterous. Thank you!
P.S. I do hope you’ll attempt them in German one of these days…
Hearing a baby laugh makes you laugh but I doubt the baby’s joke was hilarious
That’s very good. Actually babies just make me smile, but I like the way you put it.