Corn has been heavily subsidized by our government, so corporations have taken the plentiful & cheap supply and built our food supply around it. From this one product comes just about everything we eat or drink.
High Fructose Corn Syrup, what the corporations would now like to call 'Corn Sugar', is probably the most common form. This is used to sweeten everything from soda to bread to candy to cough medicine to sports drinks. You'll often find it on products that say 'Low Fat'. This is just one form it's added to food, since it is also used to do things like solidify or add texture to food (not to mention cosmetics and drugs).
Corn also feeds Cattle. This replaced their natural diet, which would includes a variety of grasses found in the field, and has reduced the nutrition they receive enough to require a constant flow of antibiotics & vitamins, just to keep the animals alive. This system allows them to live their whole life in pens crowded with other cows, ankle deep in excrement, rather than the fields.
The situation isn't much different with Chicken, Pork or even farmed fish, all of which are fed corn rather than anything resembling a natural diet. Much like cattle, what's lacking in diversity and nutrition is made up for with vitamins and antibiotics.
The effects of subsidies can be seen across the decades in rapidly rising obesity and diabetes rates:
http://www.cdc.gov/obesity/data/trends.html
Government caused this problem to solve others, and special interests making money make it hard to stop. No one with Presidential ambitions wants to anger corn filled Iowa - where the first primary elections are held.
This is also an example of where the market doesn't work, because it's a clear example where a market doesn't just produce efficiency and low price. The most important output in this market is also the food supply, which can arrive at varying levels of nutrition/good health. In this case, the market is failing.
Avoiding the corporate food system is not easy or cheap, but you aren't really living if you don't try.