CHICAGO -- Thousands of years ago, a travelling nomad put some milk in his goatskin bag and began his journey across the desert. The sun-warmed milk, agitated by the nomad's movements, was transformed by the bacteria in the bag ... and so begins the story of yogurt.

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It's a folk tale Julie Smolyansky likes to tell, as Chief Executive Officer of Lifeway Foods. The Chicago-area company is just one of the makers of cultured milk products that has seen profits skyrocket recently as Americans sweep kefir, yogurt -- and anything else with live cultures in it -- off grocery store shelves. But in the late 70s, the kefir that's now produced in huge batches at Lifeway was nothing more than a niche product, made from a recipe thousands of years old that originated in Russia. In fact, Julie's father, Mike, started the company, hoping mostly to serve Russian immigrants with a reminder of their homeland.

"It's not drinkable yogurt, " said Smolyansky, even though that's what the average shopper might think if you saw the thick, creamy stuff poured into a glass and served.

Kefir is unpasteurized milk, fermented for 18 hours and then pasteurized, or flash heated. At that point, workers add the most important ingredient: bacteria.

"It's a blend of 10 live and active cultures," Smolyansky said. She pointed to the bottle of kefir that lists the bacteria names in Latin, better read than said.

L Casei, L rhamnosus, S. Diacetylactis, B. bacterium breve ... a list of ten, and perhaps the only word you might recognize is "acidophilus." What you'll easily recognize, though, are the health claims on the bottle. They trumpet: "Enhance the immune system!" and "Promote digestive health."

That last promise is what's made Dannon's new yogurt product, "Activia" one of the most successful product launches in company history, netting more than $100 million. The television commercials say it repeatedly -- "Clinically proven to regulate your digestive system in two weeks."

And the buzz on the street? Yes, some people said, they were more regular. That was the bottom line.

And then they'd toss a scientific sounding word into the conversation -- probiotics.

It's a broad term for something the layperson would call "friendly" bacteria.

Healthy people all have it in their intestines, and it's what keeps the bad bacteria and yeast from taking over. That friendly bacteria also keeps your immune system functioning properly.

But stress, antibiotic therapy, even environmental toxins may disrupt that, and yogurt makers claim that for conditions ranging from colitis to lactose intolerance, the answer is a bowlful of yogurt or kefir. The healthy component in the creamy food is a selected few of the hundeds of kinds of good bacteria out there to rebalance your system, according to manufacturers.

At Lifeway, the specific cultures they use are so secret, we were asked not to take any pictures as they were added. And Smolyansky smilingly explained, "There are the generic, Walmart-kind of probiotics."

But Smolyansky says her company uses the "Neiman Marcus of probiotics."

Walmart or Neiman Marcus aside, do they work? Many scientific experts question it.

University of Illinois microbiologist Abigail Salyers said claims about cultured milk products are way overblown. "Right now, the overview for most scientists is that probiotics are not very effective," she said after working on the American Society of Microbiologists latest review of probiotics.

She said that among the review's findings, "There is no conclusive evidence that altering the microbiota of a healthy human individual is beneficial." And the paper even suggests there may be "potential hazards" when patients get "dose of live organisms."

"Nobody's ever died," Salyers said, "but you can have contamination with a bacterium, or virus, that can cause disease."

In fact, there have been some isolated cases of bacterial contamination in these products, but no confirmed cases of injury. And all the major manufactureres that NBC5 contacted for this story said it has never happened to them. They maintain their products are safe and good for you.

But how good are they?

Dr. Roger Clemens, a professsor and food technologist from the University of Southern California, pointed out that there's "not a regulatory statute in this country or in any country in the world" that requires makers of food with probiotics to specifically list what strain of live culture is being used. And there are hundreds of strains out there. Other experts say the labels aren't clear about the shelf life of these cultures; at some point they do become inactive. Also, there's no way to know how much or what strain to eat for which disease. In fact, Clemens said, probiotics added to food means it's not a drug. And that's why he said, "No health claims have been approved by the FDA."

The future, however, is a tantalizing one. Existing research shows that probiotics can actually communicate with different immune cells, which may modulate immune function and reduce the risk of ear and upper respiratory infections, a common problem for kids. In fact, Lifeway has created a new product called Probugs: the no-spill packaging is designed entirely with children in mind.

And last month, the company launched a first: A frozen kefir boutique in a trendy Chicago neighborhood. Lifeway is counting on the nutirtionally minded to eat up the message on probiotics when they stroll into Starfruit on Division Street.

On launch day, NBC5's Nesita Kwan visited, and one of the first customers in the store didn't want a snack or dessert. Lifting a cup to her child's mouth, Janni White said, "Kefir is supposed to be good for your body, so I thought I'd try it."