It's pasteurized - and while I agree that most ice cream machines aren't well cleaned, at a place like DQ there's enough ice cream going through it that I'm sure it's fine. Besides, I'm a big fan of everything in moderation.
I don't get the "everything in moderation" when it comes to a product that could be contaminated with Listeria. Listeria is not cumulative, you only need 1 exposure to get it. So you could eat it 200 times and not get it, or 1 time and get it.
I avoided soft serve my first pregnancy and I've been avoiding it this one as well. It's only 9 months :)
It's not that listeria is a cumulative effect, it's that everything can be contaminated with listeria.
The last big listeria outbreak in the US involved fresh cantaloupe. Also recalled in the past couple years, cake mix, popcorn, apples, onion, tofu, pasteurized cheese, veggie dip and on and on. If you start ruling out everything that could contain listeria you would be spending "only 9 months" on not much at all.