Aflatoxin is a chemical (mycotoxin) produced by a class of fungus (mold) called aspergillus.  Molds can form in the agricultural supply chain process when a crop such as wheat, soybean, or walnuts is allowed to contain too much moisture, and mold grows upon standing.  Unfortunately, these molds can be very common, and great care must be taken to make sure these farm products are free of aflatoxin.