Seeking Explanation: Food poisoning (e coli?) despite precautions

A few months back, I got food poisoning. I had sever diarrhea for 3 solid days. I determined that the only thing I ate that could have caused it was steamed asparagus & mushrooms (I suspect the asparagus). But...

I took fresh asparagus and mushrooms from the grocery store, rinsed them VERY THOROUGHLY, like I always do, and then I steamed them. I do recall that the asparagus had a funky mildew odor from sitting in a plastic bag in the fridge for a few days. But thorough rinsing and steaming should kill microorganisms. Any heat high enough to begin cooking asparagus is high enough to kill microbes. Right?

So, why did I get sick? There wasn't anything else I ate that was suspect.

(As I side note, I believe mushrooms are grown in a controlled, indoor environment, which should imply a far lesser likelihood of contamination by animal feces, than asparagus growing in an open field at ground level, where deer can poop on them, right?)