What a horrible ordeal but so thankful your wife and little boy are both well now. Yes, listeria is a nasty little beggar and not always obvious on salads.
My mother in law (bless her) was like that. We'd pop in to visit her, and she'd cleared out three or four things from her larder, and said 'These are out of date today, so you can have them'.
So, we took the items and ate/drank them over the course of the next week or so, without any harm...