Heat also breaks down the toxin that causes botulism. So, cooking food thoroughly before eating it would make it safe. Not all food can be made safe by cooking. Some harmful microorganisms, like Staphylococcus aureus and Bacillus cereus, can multiply in food and produce a poisonous substance that is not destroyed by heat. Because the poisons are already formed, nausea, vomiting, and diarrhea appear rapidly, usually 2-4 hours after eating the contaminated food.
Microorganisms are present everywhere in the soil, in the water, in the air, on our bodies.
Some are useful, like the organisms used to make yogurt, cheese, and sauerkraut. Some will cause food to spoil, making it look, taste, or smell bad. Most are harmless. But a few can make you sick. Harmful bacteria can contaminate food at any point between the farm and the dinner plate.