Milk is unhealthy because people are intolerant or allergic to it.

A significant number of people worldwide experience adverse health effects when consuming milk or dairy products. Lactose, a sugar in milk, requires the enzyme lactase in the human body to break it down. When people do not produce enough lactase, they can have lactose intolerance and have trouble digesting milk. This can lead to bloating, diarrhea, and other digestive problems when consuming milk. Some people also have a milk allergy. This is when the body recognizes casein, the protein in milk, as a threat, so it produces an immunological response to this threat, giving symptoms of allergies. This can lead to vomiting, hives, and even fatal anaphylactic shock. Casein and whey, another protein in milk, also produce hormones known to cause acne when humans digest them, and can aggravate or cause acne problems.

P[1] People are lactose intolerant or allergic to milk. P[2] Lactose intolerance or allergies to milk cause adverse health effects.