Listeria is a risk in all stages of our lives, and not just pregnancy. Its just that we are more easily able to get a touch of food poisening when we are preg. But the risks are very low. If you are going to have a chicken sandwich from somewhere - make sure the place is clean, foods are all kept cold, staff wear gloves etc, no flys - you know, just generally looks good and clean. I do avoid going to lunch bars in industrial areas (which is where i work) but, sandwich bars in citys, shopping centres etc i imainge would be fine.
I choose to avoid the risk 90% of the time - everynow and then i give in. Remember that listeria risk has only been around for about 10 or so years, so just ensure that you are eating healthy, and from decent and clean places - which i am sure you do anyway as part of your everyday eating out.