I too would handle it gently. If it's hard for you to confront her on this issue, you could say something like this. (while casually talking to her)...Goodness, son was sick last week, it really wore me out. Poor guy was so sick...I just don't know how he got sick. If she mentions her kids sick...as a possibility...say, "Oh yea, well, let's try to keep our kids away from each other when they have colds or sicknesses and such. I just don't want you guys to get sick from my guy and then you guys get sick and then we get sick again, etc. I'd feel so bad if we got you guys sick."
If she doesn't mention her kids being sick and being around your little one, then, let's say she wants to get together and your guy is sick, I'd say, "son is sick, I don't want to be around people when we are sick because that would make you guys sick and then you will be around us and then we'll get sick again and it really wipes out my energy. Are you okay with that? Thanks, I really appreciate your friendship. Your awesome!"
Anyway, tinker with it how you feel you need to. Good luck!