I wish it had been when I had my ds. I had a horribly ill intestine with a fever during labor that the attending nurse described as "stomach flu" but then I got a letter when my ds was 2 weeks old that the pasturized cheese I had been eating had been found to be contaminated w/listeria - unfortunately, at that time it was too late for a culture (as I was finally well) to verify if it had been that or something else - meaning I could not report my illness to the cheese manufacturer and they were later able to say that "no illness" had been reported from the cheese
Are you still sick? If you got over the illness without any trouble to your pg, I would stop worrying, I don't think I've read of listeria coming back or causing trouble after the initial symptoms. If you are still sick, definately check with your midwife, I believe they told me it was just a stool sample they needed for the test.
Edited to say: After reading the CDC page (linked off the above link) it seems that the hospital staff did the best thing they could by pumping me full of antibiotics (partiallly at my insistance, I was sure being that sick could not be good for my baby while I labored), and I am very thankful he came out healthy, although he was whisked to the NICU to check him over. Also that it was a blood test to look for listeria, guess I got confused with something else we thought I may have had.