"So if you're bringing in unvaccinated puppies, there's a chance that a puppy is going to come through that is incubating rabies, which is lethal for people," said Nina Marano, an animal importation expert with the CDC.
The CDC is currently proposing stricter regulations for dog imports.
"We want the puppies to have been vaccinated before they come to the United States," Marano said.
Older, female breeding dogs can be imported as well, and may end up at animal auctions.
Exclusive video from the Humane Society of the United States' undercover investigation shows auctions where breeding dogs are bought and sold.
The auctions are perfectly legal, but the Humane Society went undercover to document the conditions of some of the dogs face — cramped cages, cold temperatures, no food or water for hours on end.
"It's not about providing companionship and canine companions to American kids and adults, it's about making a profit and doing it on the backs of these poor creatures," Pacelle said.
What can you do?
The Humane Society of the United States says consider buying a puppy from an animal shelter, where one of four dogs are purebreds or a purebred rescue group.
If you still want to buy from a breeder, make sure you meet the breeder and inspect the puppies' home. If that is just not possible, request references and talk to others who have purchased dogs from that breeder.