Our Son In Law May Have Mumps. It Is His

Our son in law may have mumps. It is his daughters first birthday and we'd like to go to the party. We have both had mumps, me as a child, my husband in his 30s. However, following on from the party weekend, we are helping to look after our other grandchildren, two girls, both immunised and their 4 week old brother, born 6 weeks prematurely. He was home from hospital after 5 days, is completely breastfed and doing well. A few days ago he weighed 6lb 7oz. We are wondering how safe it will be to be with our daughter and son in law and granddaughter. As she is just one she hasn't had her mumps jab yet. Thanks for any advice you may be able to give me.