VALERIE WALBEK IS A 28-YEAR-OLD NURSE practitioner at a Falmouth clinic who gives all her pregnant patients the same advice: Eat four daily servings of dairy products and by all means avoid any dairy that is unpasteurized. That's because the US Food and Drug Administration, the Centers for Disease Control, and the American Medical Association have warned for years that unpasteurized - or "raw" - milk and cheeses can carry listeria, a potentially deadly kind of bacteria, and other pathogens that are particularly threatening to pregnant women and their babies.

But what Walbek doesn't tell her patients is that when she was pregnant with her first child last year, she drank gallons of unpasteurized milk. The milk is purchased from a Foxborough farm each week. With just a few notable exceptions - the midwife helping with the birth of her child, the Cape residents she shares milk pickup and delivery chores with, and her husband, Daniel Walbek, an engineer at Woods Hole Oceanographic Institution - she didn't confide in anyone, even though she considers the four obstetricians she works with in the Falmouth practice "all friends."

"I have mentioned to them that I go to a farm for my milk, but not that it is raw, " she says. Until now, eight months after the birth of her daughter, Lucia, Walbek hadn't revealed this information publicly. "I'm a little new to talking about it," she says.

Quietly - since the accepted medical and public health wisdom is that raw milk is a dangerous source of bacteria, including listeria, salmonella, and E. coli - hundreds of consumers around Boston have made the same decision. A total of 24 Massachusetts dairies now have permits to sell raw milk, double the number two years ago. Just Dairy, a buying club that delivers raw milk from central Massachusetts to Boston-area consumers, now drops off more than 250 gallons weekly around the metropolitan area, versus 25 gallons when it launched five years ago. Producers around the state say that raw milk is increasingly a sought-after product. Production is rising, though raw milk sells for as much as $8.50 a gallon, versus about $3.50 for pasteurized milk.

Nationwide, it's difficult to know how many people regularly consume unpasteurized milk. Selling raw milk is illegal in 18 states, and in four others, it can be purchased only as pet food. But Sally Fallon, founding president of the Weston A. Price Foundation, a nonprofit advocacy and research group in Washington, D.C., estimates (based on her organization's analysis of CDC data) that about 500,000 Americans - about 5 percent of milk drinkers - regularly consume raw milk. The group believes that the number is growing exponentially.

Until very recently, there was no such thing as "raw" milk; people have consumed milk straight from the cow for centuries. In the 1860s, French chemist and microbiologist Louis Pasteur discovered that bacteria and other harmful organisms contaminating beer or wine could be killed off by heat. The widespread pasteurization of milk starting in the 1920s "was one of the major breakthroughs in public health," says Eric Decker, a professor of food science at the University of Massachusetts Amherst. (There are several methods of pasteurizing milk before it is bottled; most commonly, its temperature is quickly raised to 161 degrees and kept there for 15 seconds.) Before pasteurization, drinking industrially produced milk in America was a gamble. In The Untold Story of Milk, Ron Schmid, a naturopathic physician and raw-milk advocate, writes that as city populations skyrocketed in the mid-1800s and pasture for cows in urban areas became scarce, dairies began feeding their cows waste grain from local distilleries. The cows quickly became diseased and emaciated, producing poor-quality milk that, coupled with inadequate sanitation and refrigeration, caused a host of health problems, mostly in young children, and created a scandal around the milk industry. Pasteurization was seen as a solution to what was known as the "milk problem."

Walbek, the nurse in Falmouth, believes it is that history and those fears that are guiding medical opinion today. "The FDA is understandably cautious in its approach," she says, but as a result, it is also "just a little behind."

SOME OF THE EVIDENCE WOULD SEEM TO back her up. The last cases of illness from raw milk recorded by the Massachusetts Department of Agricultural Resources occurred nine years ago, when 11 Boy Scouts visiting a farm became ill with salmonella after drinking raw milk; all recovered. Nationally, according to the Centers for Disease Control, from 1998 to 2005 there were 1,007 illnesses and two deaths from raw milk or cheese consumption - a tiny fraction of the estimated 76 million total cases of foodborne illnesses each year. And few foods are absolutely safe, including pasteurized milk. Massachusetts consumers found this out in December, when state public health officials revealed that three elderly men died from listeriosis they had contracted from pasteurized milk produced in Shrewsbury; a pregnant woman who contracted the illness had a miscarriage.

Even the most ardent raw-milk proponents don't suggest giving up pasteurization altogether; most just want to be able to purchase raw milk and raw-milk products everywhere, legally. "We want the choice," Fallon says. Because of high levels of disease and low levels of cleanliness, she says, her organization doesn't recommend drinking unpasteurized milk from "confinement dairies" like the ones that supply most of the commercial milk on the market. As evidence, the group points to research data such as a 2002 study conducted by the USDA's National Animal Health Monitoring System of raw milk intended for pasteurization from 860 dairies around the country. The study found a type of listeria pathogenic to humans in 6.5 percent of the dairies, and salmonella in 2.7 percent of them. Farmers who produce raw milk intended for consumers say they employ much more careful sanitation procedures to protect against such levels of contamination.

And while the FDA and others in the medical establishment argue that pasteurized milk is as nutritious as raw, many of the consumers switching to raw are swayed by new scientific findings that milk in its natural state is full of beneficial enzymes, vitamins, proteins, and bacteria - most of which are altered or killed off by pasteurization. A growing body of evidence from university research conducted around the world suggests these nutrients help counter conditions as diverse as asthma, allergies, colitis, and diabetes. A study of nearly 15,000 children ages 5 to 13 in five European countries published last year by the University of Basel in Switzerland showed that those who consumed raw milk had lower rates of both asthma and hay fever, and that the earlier in life the children started drinking the raw milk, the more effective the protection was. Results of a just-released study of 2,217 raw-milk drinkers in Michigan - conducted by a herd-share group there and by a professor at the University of Michigan and underwritten by the Weston A. Price Foundation - suggest that raw milk can be consumed by most sufferers of lactose intolerance, a condition the study's authors estimate affects about 10 percent of all Americans. This is a tiny sample, but of the 155 people in the study who said they had been "told by a healthcare professional they had lactose intolerance," more than 80 percent reported regularly drinking raw milk without symptoms. (An FDA spokesman counters that because of the study's methodologies, its authors do not consider the findings conclusive, nor do they call the consumption of raw milk a preventive measure.)

In addition to the new research, the trend is part of the broader buy-local food movement. Consumers shop at farms and farmers' markets to support local agriculture, but also to obtain foods thought to be more nutritious - harvested riper, not treated with hormones or pesticides, less processed - than what's available in many stores. In most states, Massachusetts included, unpasteurized milk cannot be sold in retail stores, or even at farmers' markets; it's only available directly from farms.

There's another interesting theory, too. The wide gulf between raw-milk proponents and opponents is part of a growing "lack of trust in contemporary institutions," says Michael Bell, a professor of rural sociology at the University of Wisconsin in Madison, who has surveyed raw-milk consumers in the Midwest. Nina Planck, the New-York-based author of Real Food: What to Eat and Why, agrees. "The same divide applies to traditional and modern medicine," she says. But she adds the debate is really heating up over pasteurizing milk for emotional reasons: "It is a children's food. This always gets people excited."

So excited that some of them defy medical advice. "My pediatrician is against us drinking raw milk at all," says Jennifer Klauder, a 30-year-old Chelsea mother of two who started drinking raw milk a few months after her first child was born. She and her husband, a computer consultant, had done plenty of research before they decided to start drinking unpasteurized milk, she says, and today, while her 10-month-old daughter doesn't drink it yet (she's still breast-feeding), her 3 1/2-year-old son does. "My family has chosen to take the risk."