If it wasn't a mean laugh, they might have been laughing that it could have been one of them, kind of a laugh of relief that the "illness monster" picked someone else so they're safe for awhile.

Maybe a snappy comeback could be, "What, you think it couldn't just as well be one of us bums?" If you include yourself it won't sound "mean", they'll know you think of yourself as one of them too, are not "better" than they are.

Or, if someone appears to be laughing "at" the other person, point a finger at them and intone in a "scary" voice, "You could be next!"

If you just don't like the laughter, then say so, "Hey guys, lets not laugh at other people's expense, we've all been where s/he's at."