A bit of misinformation here in places. Chicken Pox (varicella zoster) is at its most infectious for around 48 hours BEFORE the appearance of any spots. It usually manifests itself with a child who is off colour, a bit sniffly and with or without a slightly raised temperature two days before any spots arise.
It is infectious until all the blisters have formed scabs after around 7 days, but be aware that the spots often appear in two or three crops over a few days. The incubation period varies between 8-21 days.
In some adults the virus can be reactivated as shingles. Those on steroids, chemo and radiotherapy or who are immunosuppressive should seek GP advice if they are in contact with chicken pox.
A person with shingles can pass chicken or onto anyone who has never had chicken pox. However, you cannot pass shingles onto anyone else.