A UV filter is designed only to kill free floating organisms. It's preventative. Algae, bacteria or parasites that exist in the water column will in theory float through the UV filter sleeve, have their cellular DNA damaged beyond their ability to self-repair and then be unable to properly reproduced.
However, in the case of infections that have entered a fish's body or tissue, a UV filter isn't of much use. (Think of influenza: washing your hands and covering your mouth is effective until you actually contract influenza, then it's next to pointless.) At that point you need to identify what is the cause and take appropriate steps.
You might want to post a picture of the infection in the disease forum and get some specific advice on treatment.