The roundworm life cycle: Intestines--> blood stream--> lungs --> mature worms migrate back to the intestines.

Puppies pick them up from the mother. The Vet can will give them a puppy deworming. When your dog(s) are adults, then a once a year deworming with Panacur will keep infestations low.

P.S. With Panacur I always give them a Zantac first to keep any stomach acid reaction down.