Some women who are unable to breastfeed have turned to the Internet to buy breast milk, trusting that this is a better alternative to infant formula.

Some of the breast milk available online, however, is contaminated with bacteria, such as strep or staph. The latest study, published this month in Pediatrics Online, also found that one in ten samples bought online contained cow’s milk, which is not safe for infants.

Women who couldn’t breastfeed used to rely on wet nurses, but formula has meant that parents have another safe option. Efforts to promote breastfeeding as the best option for infants may have led some parents to believe that formula is not a good option and to turn to the Internet to find someone else’s milk, according to women’s health specialists.

Sites like Only the Breast allow people to buy, sell, or donate breast milk over the web. PennyHoarder.com suggests lactating women use their milk as a money-making project. Other sites, such as Eats on Feets and Human Milk 4 Babies, frown on making money off of breast milk; instead, they consider themselves breast-milk sharing communities and encourage free exchange after careful screening of donors.

However it is done, the FDA warns against feeding your baby another person’s breast milk.

“FDA recommends against feeding your baby breast milk acquired directly from individuals or through the Internet,” the agency said in a 2010 statement. “When human milk is obtained directly from individuals or through the Internet, the donor is unlikely to have been adequately screened for infectious disease or contamination risk. In addition, it is not likely that the human milk has been collected, processed, tested or stored in a way that reduces possible safety risks to the baby.”

Research confirms that the FDA’s warnings are sound. A 2009 study of more than 1,000 women who wanted to donate to breast-milk banks found that 36 tested positive for syphilis, hepatitis B and C, and/or HIV.

A 2013 study found that 72 percent of 100 breast milk samples bought off on the Internet contained gram-negative bacteria (which are associated with bloodstream infections, wound infections, meningitis, and fecal contamination), 63 percent tested positive for staph infections, 36 percent tested positive for strep infections, and 3 percent were contaminated with salmonella, as Rewire reported.

Though HIV can be transmitted through breast milk, that virus was not found in any of the samples.

The new study provides follow-up analysis on the same samples and found that many of them included either cow’s milk or infant formula.

Sarah Keim, the lead author on both of these studies, told CNN that the amount of these other ingredients found in the samples shows that sellers deliberately “topped off” their breast milk presumably to sell more.

“If a baby with cow’s milk allergy were to drink this milk it could be very harmful,” Keim said.

Milk banks are a safer alternative because they screen the blood of the donors and the milk itself. These banks then pasteurize the milk, which kills off contaminates. But the milk is expensive and the supply is small. Most of it goes to premature infants in neonatal intensive care units across the country.

For those who can’t produce breast milk or get it from a milk bank, formula is the best choice.

“There is a lot of unwarranted guilt surrounding not breast-feeding babies,” pediatrician Jennifer Shu told CNN. “Even though breast milk is best, in some situations, formula is the better and safer choice.”