Electron micrograph picture of Aspergillus fumigatus, courtesy of National Institutes of Health, www.niaid.nih.gov
Rice, wheat, corn and beans may carry fungal poisons called aflatoxins. Aflatoxins are naturally occurring mycotoxins produced by many species of Aspergillus fungi, due to improper storage, such as in humid conditions. Aflatoxins are a potent trigger of liver cancer and may play a role in a host of other cancers as well.
Just heating a food to the boiling point does not destroy aflatoxins. A recent study by Korean researchers showed that pressure cooking was capable of reducing aflatoxin concentrations to safe levels.
Another example is raw kidney beans, often used in making delicious Mexican Chilli. Kidney beans contain a toxin, phytohaemagglutinin, which is only destroyed by boiling at high temperature for at least ten minutes.