Clostridium perfingens likes the sort of conditions you get in casseroles, stews, gravies, pie fillings and any other bulk cooked foods when they cool. In the warm conditions of cooling food, the spores germinate and grow. Whenever you cool food, make sure you cool it quickly by transferring it into a shallow container and refrigerating it when the steam stops rising. A large stockpot, even in a commercial fridge can take several days for the centre temperature in the pot to reach 5° C. Use or freeze leftovers within two to three days.