I actually wrote my history master's thesis on Civil Rights Era Cairo, so you can imagine my surprise when I see this question on the front page! I became fascinated by Cairo's history when I participated in a photojournalism project at Southern Illinois University called the Cairo Project: http://cairo.mcma.siu.edu/ This site provides good historic background and modern human interest stories.

Looking at a map of the USA, you'd think there would be a booming city at the confluence of the Mississippi and Ohio Rivers. In the late 19th century, Cairo was a booming town, known as a railroad and river traffic hub with the untamed culture you'd expect from a northern New Orleans. Even though Cairo is in Illinois, it is the state's southern-most city and is actually further south than Richmond, Virginia. Its white-black race dynamic was as paternalistic as any in the "south," and its civil rights history was very violent. Though most people blame the violence in the 1960s and 70s for Cairo's economic decline, I found that it was really part of a general decline throughout the 20th century.
The religious element in Cairo was able to ban gambling and prostitution in the late 19th century, so part of the allure of a northern New Orleans was lost and a vibrant industry was snuffed out. Then, the decline of the railroad and river traffic industries really ruined the town. In my research I found that the economic boycott in the 60s and 70s (many white business owners chose to close their businesses and move away rather than hire black employees) was really the final death knell of a town that had already been in decline since the 1920s, well before the Great Depression.

EDIT: Fixed some minor grammatical errors. Thanks to the OP and everyone else for their kind words and interest! Nationwide, the recession has sparked interest in towns in decline and Cairo's example is the worst-case, nightmare scenario. I wish I could take this brief spotlight and direct everyone to a specific charity in Cairo working to help the city, but I haven't been there in over a year and volunteers there tend to be transient and lose faith quickly. If anyone still in the Cairo area can provide some info, please post! Also, the link above to the Cairo Project provides lots of great info on the city's past and present and was the result of the hard work of many students in the School of Journalism at Southern Illinois University and its director, Bill Freivogel.