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When Bevil Conway and his partner brought their premature twins home from the hospital, the two fathers felt it was important to keep them on a diet of breast milk . So the new parents purchased a couple of coolers and an extra freezer, and they started scouring Web sites for mothers with extra milk to share.

They found a physician who was moving away; she gave them a stash of frozen milk she had pumped but never needed. They stopped by a fire station in Lexington, Mass., to retrieve milk from a firefighter’s wife. They picked up 100 ounces from a woman whose husband wanted his freezer back before hunting season, and they made regular visits to a woman in Maine who became a close friend and produced startling, prodigious amounts of milk.

And all of it was free.

“It was amazing, absolutely amazing,” said Dr. Conway, 38, a neuroscientist and artist in Cambridge, Mass. “We managed to feed the twins continuously without any formula for 14 months.”

Wet nursing has moved into the Internet age. Where once new parents desperate for breast milk recruited a local mother or, more recently, turned to milk banks or made do with formula, now they rely on informal networks of donors, mostly strangers, hosted on Web sites like Eats on Feets and Human Milk 4 Human Babies.

But some physicians and public health experts fear that in their quest to provide infants with the benefits of breast milk, new parents may inadvertently be exposing their babies to potential harm.

Breast milk confers enormous health benefits. It’s considered the ideal nutrition for infants, and it contains antibodies and other protective immune factors that appear to reduce colds, ear and gastrointesintal infections, asthma and eczema, as well as diabetes and even leukemia, according to the American Academy of Pediatrics. Neonatal intensive care units insist on breast milk for the smallest babies because it drastically improves their prospects.

But it is also a bodily fluid that can harbor harmful bacteria and viruses, including H.I.V., and H.I.V.-positive mothers can transmit the virus to their babies through their milk.

Established human milk banks carefully screen donors, test them for diseases and pasteurize the breast milk they provide. But there is a huge demand for milk — in 2012 the banks dispensed 2.5 million ounces of milk, up from 2.1 million ounces in 2011 — and the banks must prioritize the smallest and sickest babies.

And the prices are steep. Breast milk can cost up to $5.50 an ounce, more than the cost of formula. A 3-month-old can drink anywhere from 20 to 40 ounces a day or more.

As a result, many new parents are turning to the Web, despite the Food and Drug Administration’s recommendation against feeding babies breast milk acquired directly from individuals or online.

“You don’t know what you’re getting on the Internet,” said Dr. Susan Landers, a neonatologist in Austin, Tex., one of several experts who in 2010 urged the Food and Drug Administration to step in and start regulating human milk banks. (The F.D.A. declined.)

Dr. Landers noted that even if donor mothers have tested negative for viruses and bacteria, they may drink alcohol, smoke marijuana or use medicine that can be passed on through breast milk.

When researchers reviewed the blood tests of 1,091 potential milk donors who had approached one milk bank over a recent six-year period, they found that 3.3 percent tested positive for a virus or bacterium on screening tests (some may have been false positives). Six were infected with syphilis, 17 with hepatitis B and three with hepatitis C. Six tested positive for human T-cell lymphotropic viruses (HTLV-1 and HTLV-2), and four were H.I.V.-positive.

Officials with milk banks also worry that informal milk sharing is robbing them of potential donors and could curtail the supply to premature babies. “Their lives can depend on receiving human milk,” said Kim Updegrove of the Human Milk Banking Association of North America. “And we don’t have enough. We are constantly cutting back on requests from hospitals.”

But many parents don’t even want pasteurized milk of the sort banks provide, because the heating process destroys some of the very substances — some of the milk’s immunoglobulin A, for example — that they are seeking in breast milk.

“We use it straight up,” Dr. Conway said. “We want all the antibodies.” He noted that the donors he encountered were always willing to provide their medical records and were always nursing their own babies.

So what’s a parent-to-be to do?

Pregnant women who want to breast-feed should plan for it, making sure their hospitals’ policies facilitate breast-feeding and allow a baby and mother to share a room. Pacifiers should be avoided.

If a baby is born prematurely and can’t nurse, breast milk should be pumped 10 to 12 times a day to establish a supply.

Parents who use donor milk from informal channels should ask about the health histories of the donors and for recent blood tests and medical records.

Dr. Landers suggested new parents also consider flash-heating donor milk, a technique that can inactivate H.I.V. and destroy bacteria while retaining much of the milk’s nutritional and antimicrobial properties and important antibodies.