It's not that you can't it's more that you shouldn't just in case.
But I too ate things that you should avoid, I just made sure things were fresh and if they looked dodgy or old I left it. I did feel guilty the first few times, like when I ate ham from the deli...

But I also made sure I didn't do it until I was about half way, not sure why though... Hmm! Anyway that was just me, and I'm not telling people they should go ahead and eat it but more do what they are comfortable with.