Camera Problems

Overall I've been very pleased with the camera on my iPhone; that is until about a week or so ago. Now all of a sudden my pictures are no where near as sharp or bright as they used to be. If you look at the lens it looks almost like there is something on the lens. I've tried wiping it clean with a cloth but whatever it is won't come off. Does anyone know if there is a better way to clean the lens? Any suggestions would be greatly appreciated.