Fleas are the WORST. My cats got them while we were living in a 2nd floor apartment (and my cats never go outside). And this was right after we had a bird mite infestation from a bird nest that was in our windowsill
I know it seems like you will NEVER get rid of them but if you keep up with the flea treatments for the pets they will eventually starve and die off. Also it is important to vacuum A LOT and empty the vacuum every time you do and take it outside to the trash (this helps get rid of fleas, eggs and larvae in the carpet). We also got a flea-killing spray for the carpet & furniture, you may try that.