just say "oh he can't come to school, he's sick/contagious?" if they press further, you could tell them that ds is under the care of a doctor, or is on medication, or is being treated for it, etc. and his illness is of a personal matter?
with my personality though, I would probably be open about it, oh yeah, he's got the pox, no biggie, now he'll be immune to it forever! Kids get sick. Kids get the pox. Even vaccinated kids can get the pox. To me, its no big deal, and when I was a kid, when I had the pox(15 years ago) it was no big deal. I also don't send DS to school, and we don't hang around our playgroup too much(the kids are all older than DS and usually do crafts that DS wouldn't be able to do), and the moms that I do hang around are moms who've been my friends for years before I even had DS, and if DS came down with something, pox, measles, mumps, they would say "oh no!" and be worried for DS, and they might even ask if he is vaccinated if they don't know that he's not, but i'm 95% sure that, as long as DS hadn't been around their kids in a while, they wouldn't be mad at me for putting their children in danger or anything, especially since most of them vaccinate, and if they did, I would explain to them that its very unlikely that their children would get it, and if they do, heres what to do:... but thats my personality, influenced by the relationships I have with the other moms that I spend time with.
eta: just saw your post count -- wow, you should win the major lurker's award almost a year, and this is your first post!