A food desert is a city neighborhood or other residential area in an otherwise developed nation where it is difficult to easily obtain nutritious foods. The typical example is an urban neighborhood without a major chain supermarket -- just corner convenience stores or drugstores with grocery aisles -- where residents, typically without cars because they don't need or can't afford them, find it inconvenient or even dangerous to walk to the nearest full-service grocery store. Think the little old lady who has trouble navigating unpaved sidewalks with her granny cart, or a neighborhood isolated from more prosperous surrounding neighborhoods by passenger and freight train tracks or high-traffic boulevards. Instead, the corner stores and drugstore grocery sections notoriously sell overpriced, sometimes expired foods and offer little in the way of fresh, healthy options.
Real estate developers should put supermarkets in underserved neighborhoods. I guess they don't because they've done the math and figure they can't make a profit on such an investment. I don't know all the factors they consider (I have never looked into commercial real estate development, myself), so I'm in no position to wonder just how badly a supermarket in a blighted neighborhood would actually perform. Maybe it's very difficult or very expensive to obtain insurance? There could be tax breaks, perhaps, to incentivize development; certainly it makes more policy sense, I think, than using public funds to re-route trolleys and upgrade infrastructure at a casino.
Some authorities question whether food deserts actually exist, or if the term is simply the result of circular research: a phrase that the media has picked up and repeated so often and for so long that it must be true, because it turns up in news media Nexis searches. I won't go so far as to deny the existence of urban food deserts. When I was looking for a Philadelphia home to buy in 2005, I scratched Northern Liberties and Fishtown off my list of potential neighborhoods because the closest supermarket was the Thriftway in Port Richmond. (Now, of course, there is a brand new Superfresh at 2nd and Girard.) And a good friend of mine works to attract supermarkets and other anchor retail stores to distressed communities and retain them through sustainable development practices -- he'd be the first to de-friend me on Facebook if I say, "I picked this place because the supermarket is three blocks away" in one breath, and then say, "There's no such thing as a food desert" in the next.
But consider, very especially here in Philadelphia. Every regional rail line, every subway-surface trolley, the El, and the Broad Street subway all stop within a block of the Reading Terminal Market and stop at or near the Gallery. The Market has stands offering fresh produce, fish, and meats, and the Gallery includes at least one produce market and fishmonger. I've found through my own price comparisons that the non-organic Terminal Market stands and the shop in the Gallery have prices that are competitive with local supermarkets. Most if not all of these vendors take SNAP ACCESS cards (food stamps). Now, from many places in the city, you can reach the Market and the Gallery with a single $2 transit fare (or a $1.55 token). From outlying areas in the Great Northeast, you can do it with a bus trip and a $1.00 transfer to the El. From zip code 19133, referenced above, you can do it with a single bus.
With some planning, and with adding transit fare to your food budget, you can address the food desert problem yourself, at least in Philadelphia. (This is an urban home economics blog, after all.) The transit trips may take a while, especially on weekends, so you'll need to do some time management. But if you work days in Center City yet live in a food desert, there's almost no excuse not to take advantage of the offerings in the Reading Terminal Market and the Gallery.
People who live in food deserts can't magically make commercial developers build supermarkets in their neighborhoods. But they can consider some self-reliance. Work the transit fare into your food budget. Investigate options for reduced fares or a bus pass from your employer or school. The Food Trust's Healthy Corner Stores Initiative is a great project but can go only so far; a household has to look after itself.