It's not exactly "poisoning." Corn lacks certain essential amino acids, so people who rely on corn as the staple and who don't have access to other sources of complete amino acids start showing signs of protein deficiency. This leads to a syndrome called kwashiorkor in severe cases.
You can find more info on masa and nixtamalization.at wikipedia but making corn into masa (like they do in Mexico) makes it more of a complete food. Eating plain ground up cornmeal and grits do a better job at creating that protein deficiency. This used to be a problem in the American South where poor people ate too much cornmeal because it is inexpensive