I was exactly the same, it worried me having an 8 year old getting sick and a newborn. I think you'll find he should be protected by bf. I noticed with my daughter, she only started picking things up at about 5/6 months. I guess nature designs it so it's ok, as there are heaps of families with many siblings ( i often thought about that and it made me feel better)!! Good luck, hope it goes quickly and bub is fine xoxo