How common is the flesh eating disease in urban areas?

Can happen anywhere: Flesh eating bacteria can be anywhere and that's exactly the reason for good, clean wound care. It does afflict few people, so i would not focus on that if everything else is properly done.

Uncommon: "flesh eating bacteria" are bacterial organisms that cause "necrotizing soft tissue infection". These are relatively uncommon infections, amounting to about .04% of hospital admissions. Unfortunately, they carry a high mortality rate (9 - 25%). Additionally, they create massive wounds that generally require multiple operations with reconstruction.