Dorm refrigators aren't big enough to hold a pizza box and yet college students aren't dropping dead by the dozens.
Well not dead, but they are sitting on the can.
Most food poisoning is mild and takes the form of "peeing out your ass."
Most people dont even realize that they are experiencing food poisoning.
"76 million illnesses, 325,000 hospitalizations, and 5,000 deaths in the United States each year. Known pathogens account for an estimated 14 million illnesses, 60,000 hospitalizations, and 1,800 deaths"
http://www.cdc.gov/ncidod/eid/vol5no5/mead.htm
So while eating a slice of pizza that was left out for 12 hours isnt going to kill you, it could certainly give you the shits.
There is a reason the county does annual inspections of food-service establishments. One of the main things they do is check the temperatures of the holding-bins for food that is already cooked.