Breast milk may protect children against the AIDS virus, research suggests.
Tests on mice show that even though some children acquire HIV from breastfeeding, mother's milk has a strong anti-viral effect.
Most at-risk breastfed infants do not end up with HIV despite long and repeated exposure.
Scientists carried out a study of genetically modified "humanised" mice that can acquire HIV in the same way as humans.
When the mice were given the virus in human breast milk, they were not infected.
The findings are published in the online journal Public Library of Science Pathogens.
Study leader Dr Victor Garcia, from the University of North Carolina in the US, said: "This study provides significant insight into the amazing ability of breast milk to destroy HIV and prevent its transmission.
"No child should ever be infected with HIV because it is breastfed. Breastfeeding provides critical nutrition and protection from other infections, especially where clean water for infant formula is scarce.
"Understanding how HIV is transmitted to infants and children despite the protective effects of milk will help us close this important door to the spread of Aids."