Certain metabolites of some fungi (molds) are poisonous (toxic). Fungal poisons have been known for many years, but they were not considered a major factor in animal disease until fairly recently. Because the prefix myco- refers to fungi, these toxins are termed mycotoxins. Penicillin is derived from a fungus and could be termed a mycotoxin, in relation to bacteria which are sensitive to it. There are thousands of species of fungi but relatively few of these grow on agricultural products and only a fraction are capable of producing mycotoxins. It is known that about 100 fungi which grow on standing crops or stored feeds, produce toxic substances and approximately 20 of these have been associated with naturally occurring diseases.