My friend is nursing her 7 month old since birth, is the baby safe from a sibling having chickenpox?

Yes and No: Most immunity that the baby would receive from its mother while in utero disappears by 6-8 months of age. Breastfeeding will help a baby get some protection from the mother (if the mother has been exposed to chicken pox). However, immunity to chicken pox from breastfeeding alone isn't sufficient. Infants are recommended to receive chicken pox vaccine between 12-18 months. See your pediatrician.

Do Not rely on That: Although the baby definitely gets an immunity boost from nursing, it is not a 100% guarantee for protection from specific infections. Thus one should minimize exposure, use good handwashing etc. Also keep in mind, if the nursing mom has not had a varicella (chicken pox) infection, she has no immune factors to pass thru the breast milk to the baby.

No: Any immunity to Cpx the mother may have transferred in pregnancy is gone by 6 mo and the factors in breast milk are not specific to the virus, they just generally prevent viruses from getting in.Most at that age will have a mild case (<100 spots) with incubation of 10-21 days. The kids doc could offer medicine to help with it but we seldom do in healthy kids.