Food poisoning occurs when contaminated food or water is ingested. Contamination can occur anywhere along the process of obtaining and eating food – it can occur during growing, harvesting, processing, storing, or preparation stages. In most cases, bacteria, viruses, or parasites are transferred to food from other sources, making these organisms the most common causes of food poisoning. However, in some less common types of food poisoning, the poison or toxin is naturally part of the food (e.g., poisonous mushrooms or fish). Other less common causes include shellfish and insecticides.
Bacteria and bacterial toxins: Many bacteria can cause food poisoning, either directly or by the toxins they produce. Some of the most common include Salmonella, E. coli, Shigella, Staphylococcus, Campylobacter, and Clostridium perfringens. Many bacterial causes of food poisoning can be found in undercooked meats, poultry, eggs, dairy, processed meats, fish, custards, cream pies, and contaminated water.
Viruses: Norovirus and other viruses can cause food poisoning, most commonly through contaminated raw or uncooked produce and shellfish from contaminated water.
Parasites: Parasites such a giardia lamblia can also cause food poisoning through contaminated produce and water.
Mushrooms and toadstools: Dozens of species can cause muscarine poisoning. These poisons attack the central nervous system, causing partial or complete paralysis in severe cases.
Fish: Some fish, like the puffer fish, are naturally poisonous. A poison similar to that naturally found in the puffer fish can also occur in many edible Caribbean and Pacific species. It's called ciguatera poison, and it's produced by a tiny sea parasite called a dinoflagellate. This poison attacks the nervous system.
Another kind of fish poison, called scombroid poison, is a concentrated histamine. Fish containing toxic levels of histamine often taste unusually bitter or spicy.
Shellfish: Clams, mussels, oysters, and scallops can cause poisoning when they ingest certain poisonous dinoflagellates that produce the toxin saxitoxin. This is more likely to occur in North America between June and October. Shellfish eaten during those months are potentially dangerous.
Insecticides: There are many types of poisons found in insecticides but the most dangerous types are the organophosphates, which are basically nerve gas for insects. Such insecticides are deliberately formulated to be less harmful to humans than insects, but these chemicals can be very dangerous to people if the insecticides are not used properly.
There are many other causes of food poisoning. These include wild nuts, leaves, flowers and berries, underripe tubers, botulism, cadmium from containers, lead or arsenic from fertilizers, and acids and lead from pottery.