The intact muscles of healthy livestock are generally free of microbes. The bacteria and molds that spoil meat are introduced during processing, usually from the animal’s hide or the packing-plant machinery. Poultry and fish are especially prone to spoilage because they’re sold with their skin intact, and many bacteria persist despite washing. Most of these are harmless but unpleasant. Bacteria and molds break down cells at the meat surface and digest proteins and amino acids into molecules that smell fishy, skunky, and like rotten eggs. Spoiled meat smells more disgusting than other rotten foods exactly because it contains the proteins that generate these stinky compounds.