My ds3 came down with chicken pox on Sunday. Ds2 hasn't had chicken pox yet and it is therefore quite likely that he will get it (although not definite as he has been in contact twice before and not "succumbed". The incubation period is (I think) 14 - 21 days. So he potentially becomes infectious any time between Sunday week and Sunday fortnight. Should I keep him off pre-school, swimming etc.?