My 8 year old got the flu (influenza a) when baby was almost 3 weeks old. DH got the flu the next day after 8 year old. 8 year old is still recovering, 2 weeks later. Baby and I both remained healthy. Lots of hand washing and tons of nursing were the tricks for us.
I hope your older is feeling better soon and nobody else catches it!
My ds was diagnosed with the flu on a thursday. Their pediatrician wanted to see both ds's the next day. He wanted to check out the baby and make sure he wasn't showing any signs (he wasn't- breathing was perfect, chest was clear, etc). He said at the first sign we were to call him immediately.