It might be from outdoors, grass, plants other animals. I just had a scare with fleas recently because a neighbors cat kept breaking into my yard (it had fleas) next thing you know my plants had fleas so I had to kill everything out & ban my dog from his beloved yard. Luckily I had him on frontline so it seems no fleas jumped on him. Also pet shops offer medicated grooming for them so after the yard incident I put him right to those services JUST INCASE. better safe then sorry.