Pizza can safely sit out for up to 2 hours before it can grow bacteria.
After 2 hours at room temperature the pizza can go bad because of bacteria and it may also go hard and should really be thrown out.
If you're not eating or serving the leftover pizza within 2 hours it should be refrigerated to prevent bacteria growth and then you can reheat the pizza in the oven.
Pizza is healthy for you when eaten in moderation.
However too much pizza can be unhealthy and fattening as pizza is high in fat and can cause weight gain and other health issues when consumed in excess.
Having pizza once in awhile though is healthy.
Mozzarella cheese is the most common cheese used in pizza.
However some pizza uses cheddar cheese, Gouda Cheese, Goat Cheese and Parmesan Cheese.