Milk is one of the few foods that can be a daily staple in one household and a medical emergency in the next. The illnesses linked to milk fall into four broad categories: digestive problems from lactose, allergic reactions to milk proteins, infections carried by the milk itself, and developmental risks for infants. For most people who drink pasteurized milk, none of these are relevant. For those who cannot digest lactose, who have a milk allergy, or who consume raw dairy, milk can cause real harm.

Those categories are often mixed up in public discussion. Lactose intolerance is not an immune reaction. A milk allergy is not a digestive inconvenience. A Campylobacter outbreak from raw milk has no connection to either, and a baby's immature kidneys are a separate issue. Each condition has its own mechanism, diagnosis, and management, and treating them as one condition produces bad advice.

Lactose intolerance is what happens when the enzyme runs low

Milk sugar is lactose, a disaccharide. Normally the cells lining the small intestine make an enzyme called lactase, which splits lactose into glucose and galactose so it can be absorbed. In most people, lactase production drops after infancy; the condition is called lactase non-persistence. Undigested lactose moves into the colon, where bacteria ferment it. The result is gas, bloating, cramps, and diarrhea, usually starting 30 minutes to two hours after milk or ice cream.

The scale is not small. The National Institute of Diabetes and Digestive and Kidney Diseases, part of the United States National Institutes of Health, estimates that roughly 65 to 70 percent of the world's adult population has some degree of lactose malabsorption. Prevalence is highest in East Asia and parts of Africa, and lowest in Northern Europe, where lactase persistence has been selected over thousands of years of dairy farming. A diagnosis is usually made by a hydrogen breath test, but many people with malabsorption can still eat cheese or yogurt without symptoms because fermentation changes lactose content.

Milk allergy is an immune response to the proteins

Milk allergy is one of the most common food allergies in infants and young children. Instead of lactose, the immune system reacts to proteins in milk, mostly casein and whey. The body treats those proteins as invaders and releases histamine and other chemicals. IgE-mediated reactions can appear within minutes and include hives, swelling of the lips or throat, vomiting, wheezing, and anaphylaxis. Non-IgE forms, such as food protein-induced enterocolitis syndrome, cause delayed vomiting and diarrhea.

In many high-income countries, roughly 2 to 3 percent of children under three have a milk allergy, though prevalence estimates vary with the diagnostic method. Many children outgrow the allergy, but the timing is unpredictable; some carry it into adulthood. The only treatment is complete avoidance of milk protein, which is harder than it sounds because casein, whey, and milk solids are hidden in breads, sauces, sausages, and sweets.

Confusing these two is harmless in casual conversation and dangerous in a restaurant, where the wrong order can mean an ambulance rather than a bathroom. That is why diagnosis matters before people cut a major food group out of their diet.

Raw milk carries pathogens that pasteurized milk does not

Raw milk can be contaminated with bacteria from cow feces, infected udders, milking equipment, or handlers. Campylobacter, Salmonella, Shiga toxin-producing Escherichia coli, Listeria monocytogenes, and Brucella are the names that appear repeatedly in outbreak investigations. Pasteurization was introduced precisely because these organisms spread through unpasteurized dairy.

The United States Centers for Disease Control and Prevention counted 202 outbreaks linked to raw milk between 1998 and 2018, resulting in 2,645 illnesses, 228 hospitalizations, and three deaths. That is an undercount: most diarrheal illnesses are never cultured, and not every case is linked back to a farm. The CDC's raw milk page lays out the same finding in plainer terms: raw milk is one of the riskiest foods people choose to consume.

Campylobacter is the most commonly reported cause and usually produces fever, abdominal pain, and diarrhea that can be bloody. E. coli O157 is rarer but more dangerous; about 5 to 10 percent of children with this infection develop hemolytic uremic syndrome, a condition that damages red blood cells and kidneys and can require dialysis. Listeria is the one physicians worry most about in pregnancy. Pregnant women are roughly 10 times more likely than other adults to get listeriosis, and the infection can cause miscarriage, stillbirth, or life-threatening illness in a newborn. Brucella, now rare in countries with strong veterinary controls, still causes fever and joint pain in regions where unpasteurized milk and soft cheese are common.

The public health argument against raw milk is not complicated by conflicting studies; the outbreak reports arrive with the regularity of a metronome. The issue is not that pasteurization makes milk sterile. It is that pasteurization removes the pathogens most likely to kill or hospitalize someone.

Pasteurization is a kill step, not a sterilization step

High-temperature short-time pasteurization heats milk to 72 degrees Celsius (161 degrees Fahrenheit) for at least 15 seconds. Ultra-high-temperature treatment uses temperatures around 135 to 150 degrees Celsius for two to four seconds, which is why UHT milk can sit unrefrigerated in sealed cartons. The point is not to boil milk. It is to reduce microbial load enough to prevent infection while changing flavor and nutrients as little as possible.

What pasteurization does not do is make milk permanently safe under any conditions. Pasteurized milk still needs refrigeration below 4 degrees Celsius. It can be recontaminated after heating if equipment is not clean, and pasteurizer failures do occur, though the resulting outbreaks are much rarer than those linked to raw milk. The Food and Drug Administration's questions and answers on raw milk note that pasteurized milk can sometimes spoil or become contaminated after processing, which is why use-by dates and storage conditions matter.

Milk and chronic disease: associations that are real but not the same as infection

Some studies have linked milk intake to conditions that develop over years, including acne and prostate cancer. The evidence is weaker than the evidence for raw milk infection, in the sense that association is not cause. Acne studies, often in adolescents, tend to find that higher milk consumption tracks with worse acne, and the association appears stronger for skim milk than whole milk. The American Academy of Dermatology describes the link as possible but not settled.

For prostate cancer, the World Cancer Research Fund International and the American Institute for Cancer Research classify the evidence linking dairy products to increased risk as strong. That is a weight-of-evidence judgement based on repeated findings in large cohort studies. It does not mean a glass of milk carries the same immediate risk as Listeria in pregnancy. The absolute differences in risk are modest, and men who consume a lot of dairy may differ in other ways. Nutrition researchers call this confounding; patients call it the reason no single study should change a breakfast.

The World Cancer Research Fund's prostate cancer page describes the evidence and the remaining uncertainty without pretending either is smaller than it is.

Infants under 12 months need different rules

Cow's milk does not belong in a baby's bottle. Before 12 months, the World Health Organization and the American Academy of Pediatrics recommend human milk or infant formula. Cow's milk is low in iron, and early introduction can cause microscopic bleeding in the intestinal tract, contributing to iron deficiency anemia. Its high protein and mineral load also demands more of immature kidneys than they can comfortably excrete.

This is a developmental rule, not a statement that milk is toxic. After the first birthday, whole pasteurized milk is a common part of the diet in many countries. The same product that is unsuitable at six months is ordinary at twelve.

Reducing risk without abandoning dairy

For most people, the practical advice is short. Choose pasteurized milk and dairy products, store them cold, and treat raw milk as a higher-risk food, especially if you are pregnant, under five, over 65, or immunocompromised. For people with lactose intolerance, lactose-free milk and enzyme supplements make dairy possible; hard cheeses and fermented products such as yogurt are often easier to tolerate. For people with milk allergy, no amount of processing helps, because heating does not reliably destroy the allergenic proteins.

• Keep pasteurized milk below 4 degrees Celsius and follow use-by dates.
• If you are pregnant or have a weakened immune system, avoid raw milk and soft cheeses made from unpasteurized milk.
• For lactose intolerance, try lactose-free milk or yogurt and hard cheese, which contain less lactose than fluid milk.
• For milk allergy, read every label; casein, whey, and milk powder appear in foods that do not taste milky.
• When traveling where unpasteurized dairy is common, ask whether milk and fresh cheese were boiled or pasteurized.

The question "what illnesses can milk cause?" has a less satisfying answer than yes or no. It can cause digestive symptoms, allergic reactions, and bacterial infections, but those risks are distributed very differently across the population. The remaining public health question is why the version of milk most likely to send someone to hospital is still sold under the word "natural."

Photo by ROBIN WORRALL on Unsplash