An effective one what? A fruit fly trap? A basic one is just a plastic bottle with holes punched in the side and some liquid bait at the bottom. This isn't to catch fruit flies alive and feed to your amphibians, but to help control your problem. Good sanitation and secure breeding containers for your feeders is most important though.
I can't see how getting sick from fruit flies would be possible. Don't try to capture and breed the ones that fly around and invade your kitchen, if you want to raise fruit flies read this Frog Forum - Fruit Fly Culturing, Care and Feeding - Everything you need to know about Drosophila