Fleas don't live on the dog. Yet one or two can bite, and cause real irritation. I liken it to those with sensitive skin who go through hell when bitten through that awful itching. When we had a very minor flea outbreak, (it probably wasn't minor at all....) but God, it really caused problems for Casper. It took ages to work it out, and in that whole time I never saw a flea. Not even when we bathed all three dogs, and checked the debris under a microscope. It was fleas though, when we treated all dogs, cats and furniture the scratching magically disappeared.