They are called "puppy mills" -- where scores of puppies are crammed into tiny, filthy cages and eventually sold to unsuspecting dog lovers.

"There are two to five million puppies being churned out by puppy mills every year," says WAYNE PACELLE, President of the Humane Society of the United States, which recently conducted a five-month investigation that turned up more than 900 puppy mills in Virginia alone.

"We found only 16 of the operations in Viriginia that were actually registered and licensed by the U.S. Department of Agriculture," says Pacelle, adding that unscrupulous breeders often produce dogs with mental and physical defects.

Which is one of the main reasons why the Humane Society is reminding consumers to adopt their pets from shelters and rescue centers.

When we showed "Dog Whisperer" CESAR MILLAN exclusive video of one of the puppy mills, he said, "It's a sad, sad reality that in such an incredible country there exists something like this."

And while many Hollywood stars -- from OPRAH WINFREY to ELLEN DeGENERES -- get their pooches from reputable breeders or save them from shelters, Milan says these poor pups can end up anywhere, including celebrity homes.

"Many times," says Millan, "people who purchase the dogs don't know where they come from."