1. When you consume pizza that has been sitting on the counter overnight, you consume the bacteria that's been growing on it too. This increases your chance of contracting a foodborne illness–or getting what many people commonly refer to as food poisoning.
2. Yes, if you are eating raw mushrooms!
Raw mushrooms can be a great addition to your diet. They have been shown to contain certain minerals like vitamin D and iron and help build immunity. However, they should not be eaten raw. If you decide that you would like to enjoy them this way, wash them well first. Then cut into pieces and sautee them in a pan with some olive oil. You may eat them right away, but it is better to cook them before consuming.
3. No, you can still get sick after eating leftover pizza
This question was asked by a student from my cooking class at NYU. Pizza is basically bread dough that has been spread with tomato sauce and cheese. Leftover pizza should be kept covered until it cools down completely to prevent bacteria from multiplying. There's no specific time period for how long it needs to sit out, however, if you're worried about getting sick, you could put the pizza back in the fridge overnight.
However, it is unlikely that any symptoms of illness would occur when eating pizza the next day.