Staphylococcus aureus is commonly found on skin. FoodSafety.gov notes that people who carry the bacteria can contaminate food if they do not wash hands before touching it.
Staph food poisoning often begins within 30 minutes to 8 hours with nausea, vomiting, and stomach cramps. Cooking may kill bacteria but not destroy toxin already formed.
Keep hot foods hot, cold foods cold, and refrigerate leftovers within two hours.