How does flea prevention medicine kill all the fleas if its only on the back of the pets head?

You know when you put that flea medicine behind your pets head how does that kill all the fleas on the entire body of the animal.How does flea prevention medicine kill all the fleas if its only on the back of the pets head?The medicine mixes in with the natural oils on the animal's skin and is spread everywhere. That's why you don't want to bathe an animal for at least 48 hours after treatment.How does flea prevention medicine kill all the fleas if its only on the back of the pets head?flea medicine goes in to the blood stream of the cat and dog that's how it kills the fleas within a few hours also that's why you have to comb them to get rid of the flea dust.