How Big Do Foreign Invaders Have To Be To Cause A Reaction From The Immune System?

I mean, are we talking microscopic here or bigger? Or does it all depend on the type of bacteria?

Why do I ask?

Well, ever since my “diagnosis,” I’ve developed a pathological fear that I am going to get sick. If someone coughs, I am going to get a cold. If someone throws up, I am going to get the flu. That is, if they are sick when they are around me. I don’t just hear that someone is sick and think I am going to get sick.

There are several reasons for this:
1) All of this started, or domino-ed, from a few innocent Strep throat infections.
2) My immune system is already compromised from my illness.
3) My immunosuppressive medications further compromise my already compromised immune system.

What’s a girl to do? Stay at home and become a hermit? Where a mask? Hope for the best?