Women who wish to breastfeed their children had better think twice about seeking an outside supply of it from the Internet. According to a new study from Ohio’s Nationwide Children’s Hospital, more than 10 percent of breast milk sold online is mixed with cow’s milk. Researchers say they believe the purity has been compromised to increase the volume of milk to boost sales. Roughly 20 percent of milk acquired for the study tested positive for cytomegalovirus, which can cause illness in sick babies. Doctors fear donors who are not properly screened may be introducing harmful drugs or alcohol into the milk supply.