Aflatoxins are among the most carcinogenic substances known. When the liver is exposed to them, necrosis, cirrhosis, and/or liver cancer can result--though I don't know the whole process, not being a medical type. :-)
I know this is a problem with peanuts, having worked at the pb cup factory years ago, but it affects other crop types as well.
IIRC p53 is one of the normal repair systems within cells, so that it can be considered a kind of braking system against uncontrolled growth. p53 mutations can interfere with this natural braking mechanism. Though, generally speaking this alone will not be sufficient to induce cancerous growth, when combined with the breakdown of other braking mechanisms, such as apoptosis, and the upregulation of certain growth signaling factors (either through genetic bad luck, or mutation resulting from environmental factors), trouble can result.
So, which is worse - moldy peanuts and liver cancer, or moldy grain and being condemned as a witch?