Mycotoxins occur naturally in some mushroom species and marine biotoxins in some shellfish and fish species. Apart from being responsible for crop losses, certain mycotoxins such as aflatoxin, ergot and "yellow rice" result from growth or storage of crops under conditions leading to infestation by fungus, which produce highly toxic chemicals, frequently invisible, that can not be destroyed by normal processing and cooking. With the rapid expansion of fisheries as a means of providing a source of protein (particularly in developing countries), there is increasing concern over the presence of toxins in fish. The hazard of marine biotoxins is particularly insidious because species of fish that have been eaten for many years without danger may suddenly become poisonous, causing many deaths.
An American toxicologist calculated that a person can consume 1.5 grams of natural toxins a day through foods such as coffee, tomatoes, wheat, rice and potatoes -- about 10,000 times the average amount of pesticide residues consumed. Half of the 52 natural toxins that were tested were carcinogenic.