Aflatoxins are one of the most potent and dangerous groups of mycotoxins worldwide. Over four billion people in developing countries are repeatedly exposed to aflatoxins, contributing to greater than 40 percent of the disease burden in these countries. Aflatoxins are produced primarily by the fungi Aspergillus flavus and Aspergillus parasiticus. There are four main types of aflatoxins: B1, B2, G1, and G2. Aflatoxin B1 is the major toxin produced, and is regulated in the United States at 20 ppb in agricultural products that may be used in human food. The clinical effects of aflatoxins may include death, liver cancer, reproductive problems, anemia, immune system suppression, and jaundice. Nursing animals may be severely affected by a toxic derivative of aflatoxin (aflatoxin M1) that can be passed through milk.