Olive oil I'm pretty relaxed when it comes to food. I was very good for the first trimester with DS but then pretty much ate normally. I only avoided the big risks such as cold rice, alcohol, caffeine and anything from liver such as pate (too high in vitamin A). Apart from that I just made sure I ate things fresh. For example most soft cheese is pasteurised so I just made sure it was from a fresh packet and not one that had been open a while. The risk is fairly low for getting listeria. I still eat ham, have my steaks medium rare etc but I have a good butcher where I'm comfortable everything is really fresh and have it on the same day we buy it.
In terms of party leftovers though, they could have been cut and left out at room temperature for quite a long time before being wrapped up and refrigerated again. This is a tough one as you don't really know how fresh it still is, though you'd get a pretty good idea but looking at it I guess!