I'm like you...never had a flea problem with any of my dogs...ever. I don't use chemicals, monthly pills or topical liquids...we've just never had fleas.
But, we did end up with a flea issue a year or so ago when we took in a little stray and he was covered in them so, of course, all of mine ended up with them too.
I tried the topicals...didn't work
I tried the 'natural' shampoos and sprays...didn't work
I tried flea shampoo...didn't work
I finally got a pill from my vet...Capstar I think...and, in combination with daily baths in Dawn dish soap...it worked
No more fleas and my little crew was happy again
The little stray left and we went back to our 'normal' routine of no chemicals and we haven't had a flea since.