A small marketplace for breast milk has sprouted online, but a study published Monday confirms what health professionals suspected: Internet breast milk is often tainted.

The study, published in Journal of Pediatrics, found that breast milk donated or sold on two popular websites was often contaminated with high levels of bacteria that could make a child sick, including salmonella. Sixty-four percent of the samples were contaminated with staph, 36 percent with strep and almost 74 percent had so much bacteria that they would have failed the Human Milk Banking Association’s criteria, the Associated Press reports.

Recent research has found that breast milk protects infants from infections, so doctors have been encouraging new mothers to breast-feed rather than use formula. But not all women can do so — couples who have adopted, mothers who have mastectomies and women who simply cannot produce enough milk are forced to rely on donated or purchased breast milk.

Some turn to breast milk banks, where the Human Milk Banking Association screens donors. But banks prioritize premature infants with medical complications, and do not hold enough milk for healthy infants whose mothers simply cannot lactate.

In 2011, there were more than 13,000 postings on the four leading milk-sharing sites, the AP reports.