A handful of infections can genuinely pass from mother to infant through breast milk, but the list is shorter than many people assume. HIV and a related virus called HTLV-1 are the clearest examples of pathogens that transmit this way with meaningful frequency. Cytomegalovirus (CMV) commonly shows up in breast milk too, though it rarely causes problems except in very premature babies. Beyond these, the picture gets murkier, and for several infections that worry new parents, the evidence points toward breast milk being safe or even protective.
HIV and Breastfeeding
HIV is the most studied and most consequential infection transmitted through breast milk. Despite major progress in reducing mother-to-child transmission overall, breastfeeding still accounts for roughly half of new pediatric HIV infections recorded each year worldwide.1PubMed Central. Mother-to-Child Transmission of HIV Through Breastfeeding Improving Awareness and Education: A Short Narrative Review The virus is present both as free-floating particles in breast milk and inside infected cells, and even antiretroviral treatment does not completely clear it from the milk. Research has shown that while certain drug regimens suppress the free-floating virus in milk, the level of HIV-infected cells remains similar regardless of which treatment a mother receives.2PubMed Central. HIV-1 persists in breast milk cells despite antiretroviral treatment to prevent mother-to-child transmission
That said, the risk is not the same for every mother with HIV. When a mother takes antiretroviral therapy consistently and maintains an undetectable viral load, transmission through breastfeeding becomes very low. This shift in evidence has reshaped official guidance. U.S. federal guidelines updated in 2023 now support shared decision-making rather than a blanket recommendation against breastfeeding. Mothers with HIV who are on effective treatment and have consistent viral suppression can discuss formula feeding, banked donor milk, or breastfeeding with their healthcare providers, and receive nonjudgmental support for whichever option they choose. Antiretroviral prophylaxis given during pregnancy and breastfeeding has been shown to reduce HIV transmission and the combined outcome of transmission or death at 12 months compared to shorter treatment courses.3Cochrane Database of Systematic Reviews. Antiretroviral prophylaxis for preventing mother-to-child transmission of HIV through breastfeeding
HTLV-1 and the Duration Question
Human T-lymphotropic virus type 1, or HTLV-1, is a less well-known but important pathogen that spreads through breast milk. This virus can cause a rare and aggressive form of leukemia or a progressive neurological disease decades after infection. What makes HTLV-1 transmission through breastfeeding distinctive is how strongly it depends on how long a mother nurses. A meta-analysis found that breastfeeding for more than six months roughly quadrupled the odds of passing the virus to the infant compared to bottle feeding. But breastfeeding for six months or less showed no statistically significant increase in transmission risk at all.4PubMed Central. Human T-lymphotropic virus type I and breastfeeding; systematic review and meta-analysis of the literature
This duration effect matters practically. In regions where HTLV-1 is common, such as parts of Japan, the Caribbean, and sub-Saharan Africa, limiting breastfeeding to six months or switching to formula can substantially reduce transmission without forgoing the early benefits of breast milk entirely. In countries where the virus is rare, routine screening of mothers is not standard practice, though milk banks do screen donors in some settings.
Cytomegalovirus and Preterm Infants
CMV is one of the most common viruses found in breast milk. The majority of adults have been infected at some point, and the virus reactivates in the mammary gland during lactation. For healthy, full-term babies, acquiring CMV through breast milk is generally considered harmless.5Nature Communications. Human cytomegalovirus in breast milk is associated with milk composition and the infant gut microbiome and growth Most of these infants either fight off the virus without symptoms or develop mild, self-limited illness.
The concern is premature babies, especially those born very early. In a study of preterm infants who acquired CMV through breast milk, those born at around 30 weeks’ gestation and infected after eight weeks of age tended to have mild or no symptoms. But extremely low birth weight infants born at around 24 weeks were infected earlier, between four and seven weeks of age, and most of them developed marked symptoms of acute CMV infection.6PubMed. Transmission of cytomegalovirus to preterm infants through breast milk Some research has linked breast-milk-acquired CMV to brain changes and neurological problems in the most vulnerable preterm infants.7PubMed Central. Human Breast Milk-acquired Cytomegalovirus Infection: Certainties, Doubts and Perspectives
This creates a difficult clinical tradeoff. Breast milk provides enormous nutritional and immunological benefits for preterm babies, who desperately need both. Some neonatal units freeze or pasteurize the mother’s milk to inactivate CMV before giving it to very premature infants, though freezing only partially reduces viral load. The decision often comes down to how premature the infant is and whether the mother is CMV-positive.
Hepatitis B and C Are Safer Than Many People Fear
Hepatitis B is one of the infections that generates the most anxiety around breastfeeding, and the evidence is reassuring. Multiple studies have found that breastfeeding carries no additional risk of hepatitis B transmission to infants, as long as the baby receives proper immunization at birth. A meta-analysis concluded that without cracked or bleeding nipples or lesions, breastfeeding did not contribute to mother-to-child transmission of hepatitis B after the infant received appropriate immunization, and should be recommended as a valuable source of nutrition.8JAMA Pediatrics. Breastfeeding of Newborns by Mothers Carrying Hepatitis B Virus: A Meta-analysis and Systematic Review Separate reviews have reached the same conclusion, finding no differences in hepatitis B transmission based on whether mothers breastfed or not.9PubMed Central. Breastfeeding and chronic HBV infection: clinical and social implications
The situation with hepatitis C is similar. While the virus has been detected in breast milk in some studies, no strong evidence demonstrates that breastfeeding transmits it. Major medical organizations generally do not advise against breastfeeding for hepatitis C-positive mothers, though they recommend stopping if nipples are cracked and bleeding. Milk banks, on the other hand, take a more cautious approach and exclude donors who test positive for either hepatitis B or C, partly because data on whether standard pasteurization fully inactivates these viruses remain incomplete.10PubMed Central. Recommendations for the establishment and operation of a donor human milk bank – Section: Bacteriologic screening and quality control
Emerging Viruses and the Evidence Gap
Every new viral outbreak raises the question of whether the virus can spread through breast milk. The answers tend to follow a pattern: the virus is detected in milk, headlines erupt, and then further investigation reveals that detection alone does not mean transmission.
Zika virus is a clear example. During the 2015–2016 outbreaks, researchers found Zika RNA in the breast milk of infected mothers. But a systematic review noted that the data were insufficient to conclude that Zika actually transmits through breastfeeding. In the cases examined, infants tested positive so soon after birth that they were probably infected in the womb or during delivery rather than from milk.11PubMed Central. Transmission of Zika virus through breast milk and other breastfeeding-related bodily-fluids: A systematic review Another study found that none of the breastfed babies of Zika-infected mothers developed the disease, even when viral particles were isolated from the milk, suggesting that the presence of infectious virus in breast milk may not be enough for efficient transmission.12PubMed. Zika virus shedding in human milk during lactation: an unlikely source of infection?
COVID-19 followed a remarkably similar script. SARS-CoV-2 RNA was found in breast milk samples from some infected mothers, but the vast majority of samples tested negative. Critically, when researchers looked for subgenomic RNA (a marker that the virus is actually replicating and could infect cells), they found none. In a study of 110 lactating women with COVID-19, no infectious virus was detected in any breast milk sample.13PubMed Central. No Evidence of Infectious SARS-CoV-2 in Human Milk: Analysis of a Cohort of 110 Lactating Women A living systematic review concluded that the few infants who did test positive for COVID-19 after breastfeeding were more likely exposed through close contact with infected family members rather than through the milk itself.14PubMed Central. Transmission of SARS‐CoV‐2 through breast milk and breastfeeding: a living systematic review Major health organizations continued to recommend breastfeeding throughout the pandemic.
Ebola presents a different scenario. The virus has been found in breast milk of recovering patients, and unlike Zika or COVID-19, Ebola is known to persist in various body fluids for months after recovery. In outbreaks, mothers who have survived Ebola are typically advised to avoid breastfeeding until their milk tests negative, though the practical infrastructure for testing is often limited in affected regions.
Bacterial Infections Rarely Transmit This Way
Bacterial infections spreading through breast milk are uncommon. A review of the evidence concluded that maternal bacterial infections are rarely complicated by transmission to infants through breast milk. In a small number of situations, a temporary pause in breastfeeding is recommended: about 24 hours for infections like gonorrhea, Haemophilus influenzae, Group B streptococcus, and staphylococcal infections, and longer pauses for others including Lyme disease, syphilis, and tuberculosis. In some cases, the infant may also receive preventive antibiotic treatment.15PubMed Central. Breast milk and infection
The more common bacterial issue with breastfeeding is not transmission of a systemic disease but localized problems like mastitis, where bacteria infect the breast tissue itself. This can introduce bacteria into the milk, but continuing to breastfeed is usually recommended during mastitis treatment because stopping can make the infection worse by causing milk to back up.
Candida, a yeast rather than a bacterium, deserves a mention here too. Thrush can pass back and forth between a baby’s mouth and a mother’s nipples during nursing. This is not a breast-milk-transmitted disease in the traditional sense; it is a surface infection spread by direct contact. Still, healthcare providers often treat both mother and baby simultaneously to break the cycle.16PubMed Central. Nipple candidiasis among breastfeeding mothers. Case-control study of predisposing factors.
Chagas Disease and Parasitic Transmission
Chagas disease, caused by the parasite Trypanosoma cruzi, is endemic in parts of Latin America and is increasingly seen in non-endemic countries through migration. Researchers have detected T. cruzi DNA in the breast milk of infected mothers and in the blood of their newborns, which raises the possibility of transmission through nursing. However, proving that breastfeeding is the route of transmission rather than exposure during delivery has proved difficult. One study of Mexican mothers with Chagas disease found parasite DNA in both breast milk and newborn blood but could not determine whether the parasite reached the infant through nursing or through the bloodstream during birth.17PubMed Central. Trypanosoma cruzi DNA Identification in Breast Milk from Mexican Women with Chagas Disease Current guidance generally advises that mothers with Chagas disease can breastfeed unless they have acute infection or cracked, bleeding nipples.
Environmental Contaminants Are a Different Kind of Exposure
Not all harmful substances in breast milk are infectious. Persistent organic pollutants, including compounds like polychlorinated biphenyls and certain pesticides, accumulate in body fat over years and are gradually released into breast milk. A scoping review of recent data found that these pollutants appear in higher concentrations in breast milk the longer lactation continues, and that contaminants accumulated in a mother’s fat tissue can be excreted into milk years after the original exposure.18PubMed Central. Pollutants in Breast Milk: A Scoping Review of the Most Recent Data in 2024 Breast milk has been used as a monitoring tool for environmental pollutant levels in populations for over 70 years.19PubMed Central. Application of the AI-Based Framework for Analyzing the Dynamics of Persistent Organic Pollutants (POPs) in Human Breast Milk Studies from places as varied as Austria and Croatia have detected flame retardants and per- and polyfluoroalkyl substances (PFAS) in virtually all breast milk samples tested.20PubMed Central. Persistent Organic Pollutants in Austrian Human Breast Milk Collected between 2013 and 2016
This sounds alarming, but context matters. The concentrations detected are typically very low, and every major health organization still considers the benefits of breastfeeding to outweigh the risks from trace contaminants for the vast majority of mothers. The concern is primarily a public health one about reducing environmental exposures at the population level, not a reason for individual mothers to stop breastfeeding.
Medications present a related but distinct question. Drugs pass into breast milk through a variety of mechanisms, and predicting how much of any given medication will end up in the milk is more complex than simple rules of thumb suggest. A recent study found no straightforward molecular-weight cutoff that predicts whether a drug will enter breast milk, challenging common clinical assumptions. While having a drug bind strongly to blood proteins (above about 75%) can be a useful initial indicator that less will reach the milk, this rule breaks down when the mother clears the drug slowly or when the drug has active byproducts.21PubMed Central. Rules of Thumb for Estimating Drug Levels in Breast Milk: How Well Do They Work? Databases like LactMed exist specifically to help mothers and clinicians check individual medications.
Why Breast Milk Also Protects Against Infection
The conversation about disease transmission through breast milk exists against a backdrop that often gets overlooked: breast milk is one of the most potent sources of immune protection a newborn receives. It contains antibodies, immune cells, and a range of bioactive molecules that actively defend the infant against infections and help shape the developing immune system.22PubMed Central. Human Breast Milk: From Food to Active Immune Response With Disease Protection in Infants and Mothers Breast milk antibodies do not just passively block pathogens. They help prepare the newborn for environmental exposures, guide the infant’s immune development, and may even promote the establishment of healthy gut bacteria.23PubMed Central. Antibodies in breast milk: Pro-bodies designed for healthy newborn development Evidence suggests these antibodies play roles in both preventing infection and supporting the selection of beneficial microbes that colonize the infant gut.24PubMed. The multifaceted roles of breast milk antibodies
This dual nature of breast milk, carrying both protective factors and, in certain cases, infectious agents, is what makes clinical decisions so nuanced. For most infections, the protective effects of breastfeeding outweigh the transmission risks. The exceptions, HIV and HTLV-1 in particular, are situations where the specific pathogen has evolved to exploit the very intimacy of the breastfeeding relationship to spread efficiently.
How Milk Banks Handle the Risk
Donated breast milk goes through safety steps that eliminate most concerns about disease transmission. Milk banks screen donors for infections including HIV, HTLV, hepatitis B and C, and syphilis. Donated milk is then pasteurized using the Holder method, which involves heating to 62.5°C for 30 minutes. This process inactivates nearly all viral and bacterial infectious agents, including HIV, HTLV-1 and HTLV-2, and CMV.10PubMed Central. Recommendations for the establishment and operation of a donor human milk bank – Section: Bacteriologic screening and quality control Even though pasteurization is effective against these viruses, milk banks still exclude HIV-positive and hepatitis-positive donors as an extra precaution.
Pasteurization does have tradeoffs. It reduces some of the bioactive and immune components that make breast milk valuable in the first place. But for babies who cannot receive their own mother’s milk, particularly premature infants in neonatal intensive care, pasteurized donor milk remains far preferable to formula for many outcomes. The layered approach of screening plus pasteurization plus bacteriological testing makes the risk of disease transmission through banked milk vanishingly small.
Informal milk sharing, where parents obtain breast milk from friends, online communities, or other unregulated sources, does not carry these safeguards. The milk is unpasteurized, the donor is unscreened, and contamination during collection and storage is possible. For families considering informal sharing, the risks are meaningfully higher than with milk from an accredited bank.