A toxin is a poisonous substance produced by living organisms, for example toxins can be produced by the metabolic activities of some bacteria or carried by fish or released by certain plants. If bacteria present in food release toxins, the food can become unsafe to eat. Examples: Botulism caused by the toxin from the bacterium Clostridium botulinum and scombroid poisoning from the naturally occurring scombroid toxin in some improperly refrigerated fish, such as mackerel and tuna.