Mycotoxins are a major threat to public health in many 'developing world' countries as storage of corn can be quite poor or inconsistent - in 2007 alone there were over 100 deaths due to eating contaminated food. In countries such as EU and USA where mycotoxin levels are closely monitored this isn't a problem.
"Environmental factors like drought, high temperatures, nitrogen availability and insect damage in plants allow the fungus to thrive. Fungal spores can enter the corn via cavities created by insects, and later germinate and produce mycotoxins, the problematic family of contaminants that includes aflatoxin."
Monsanto have developed genetically manipulated corn that is designed to ward off insect attack, and have found that the same strains of corn are also less likely to accumulate some mycotoxins, presumably because they have less insect damage. They plan to further develop this as a useful feature for their crops.