For many people, pets are considered loved members of the family.

However, many families are losing their furry little loved ones to deadly illnesses, according to a 10News investigation.

The puppies are cute and cuddly, and they are sold to Americans by the carload on the streets of Tijuana, Mexico.

Many of these dogs are too young to leave their mothers and are dehydrated and sick.

"That's the type of puppies and the condition of the puppies that are being sold on the streets,” said Ginny Bischel, a doctor of veterinary medicine.

Bischel agreed to go undercover with 10News investigators to look for sick puppies.

A former Baja resident took 10News to Tijuana’s notorious “mile of puppies,” where generations of puppy peddlers sell young dogs.

Sellers line up along the boulevard peddling puppies of any breed one can imagine.

A California man on the street offered a boxer puppy for sale. For a purebred boxer, it was offered at a good price.

However, was it really a value or was it another sick dog?

Not far from the mile of puppies was a breeder who sellers told 10News is a major supplier of dogs sold on the streets.

10News’ hidden cameras showed just how bad the conditions are.

There were puppies jammed into small kennels, and, in some cases, more than a dozen dogs per cage.

There were pinschers, poodles and schnauzers all crammed together with no sign of water or food.

The owner became suspicious of the cameras and told 10News to leave.

The owner’s wife asked 10News investigators to get out and for her husband to get a pistol.

Back at the mile of puppies, a seller admitted to 10News that many -- if not most -- of the dogs sold on the streets and at Tijuana’s puppy mills are very sick.

The journey for many of the puppies starts with a two-hour flight from Guadalajara, Mexico, to Tijuana. Sometimes there are hundreds of dogs on every flight.

Many of the dogs are dehydrated, have worms or infectious diseases.

The vast majority of the puppies come from Guadalajara, while others are bred in the back yards of Mexico City and Leon, Mexico, and are flown to border towns.

Bischel said, “They get them by the hundreds and everyone is selling them on the streets. That's where they get them from."

Tijuana’s street trade is not the only destination for the Mexican puppies.

Thousands are smuggled across the U.S.-Mexico border every year, jammed into suitcases or stuffed into kennels.

"We're very concerned about parvo and distemper and rabies coming into the United States,” said Vince Bond of U.S. Customs and Border Protection.

K-9 teams at the border search for smuggled puppies and the cars of repeat offenders are confiscated, but the practice continues.

Bischel added, “It will not stop. As long as people are buying these puppies, they will continue to breed."

10News learned from investigators that puppy sellers will give buyers copies of veterinarian bills and shot records -- most of which are documents that most of the are considered counterfeit or signed by veterinarians who were paid off by the puppy peddlers.