The milk you buy in the grocery store (including organic milk) has been pasteurized, the process by which milk is heated to a specific temperature in order to destroy harmful bacteria. The process was named after Louis Pasteur. Pasteurization does not reduce the nutritional value of milk; it still contains the same amount of calcium and other nutrients.
It's not safe to drink raw milk, even from dairy farms where the producers take extra safety precautions. Raw milk can be dangerous for your health because it may carry bacteria that can make you sick. In fact, in the United States, it's illegal to transport and sell raw milk across state lines, although some areas allow raw milk to be sold locally. You need to check the laws in your state if you're interested in purchasing raw milk.
Raw milk is especially dangerous for infants, young children, elderly people and those with immune system problems. Symptoms of illness include nausea, vomiting, diarrhea, headaches, abdominal cramps, fever and body aches.
Some raw milk proponents make the claim that pasteurization kills natural enzymes in milk (it does) and that those enzymes have healing properties (there's no scientific evidence supporting that idea). There's no credible evidence to back up the claim that raw milk is better for you because of anything the enzymes do in your body -- the enzymes are digested just like any dietary protein.
Another claim is that raw milk is less allergenic. But pasteurization doesn't have an effect on milk proteins or milk sugars, so drinking raw milk doesn't relieve milk allergies or help lactose intolerance. People who can't drink milk due to allergies or intolerance will have to avoid raw milk and pasteurized milk.
Women who are pregnant or breastfeeding should not consume raw milk or certain types of soft cheese. Pregnant women can still eat hard aged cheese, cottage cheese and processed cheese. According to the U.S. Center for Disease Control:
"Avoid soft cheeses such as Feta, Brie, Camembert, blue-veined, and Mexican-style cheese. (Hard cheeses, processed cheeses, cream cheese, cottage cheese, or yogurt need not be avoided.)"
These soft types of cheese may be contaminated with listeria, which can cause serious infections in pregnant women, newborns and people with compromised immune systems.
Centers for Disease Control and Prevention. "Listeriosis." Accessed June 29, 2010.
Ohio State University. "Health Benefits, Risks, and Regulations of Raw and Pasteurized Milk." http://ohioline.osu.edu/fse-fact/pdf/0003.pdf Accessed June 29, 2010.
United States Food and Drug Administration. "Milk, Cheese, and Dairy Products - Myths About Raw Milk." Accessed June 29, 2010. http://www.foodsafety.gov/keep/types/milk/index.html.