If I got an email like that from a teacher that specifically mentioned the flu, I would think that the teacher was giving parents a heads up that their child has most likely ALREADY been exposed to the flu. The teacher may very well have caught it from a student. He may have wanted parents to be on the look out for flu symptoms in their kids. He didn’t have to share that info (and he’ll think twice in the future, I bet.)
If you and your DH are going to stress over germs, you are going to be stressed all the time. I think that’s why PP are suggesting working in your own response- it’s the one thing in your control. (And lots of hand washing!) As you are finding, kids get sick in classrooms. This is normal. They may start off fine in the morning, and then throw up or get a fever. They may cough, and sneeze, and pick their boogery noses and touch things. You could try homeschooling, but you’d have to stay in your house because people in public do all this stuff too. I bet though with time you’ll start to adapt to this as your kid will get older and she’ll probably start, like most kids, to get a stronger immune system. That’s the positive side to the exposure!