That's not very considerate of your sister!  You'd think she'd no better having been through pregnancy herself!  DS had this a while back while a friend of mine was pg and I freaked out and stayed away from her for weeks!  I spoke to my dr about it later, bc DS is in creche and I was wondering how to manage the risk of this and 100 other childhood illnesses for me when we were planning #2.  He didn't seem to bothered.  I *think* it was only a problem if you caught it and apparently it's really uncommon in adults.  So hopefully you'll be ok.  MAybe tell your MW, just so she can look out for anything?  GL
