40% of adults are lactose intolerant to some degree. Another significant percentage are allergic to cow's milk. I'm in that latter group, and can't go near the stuff without winding up with puffy skin, horrible digestive problems and breakouts that resemble acne.
Also, studies have shown a 40x increased risk for leukemia in children who consume cow's milk. This is likely due to the fact that 90% of cow herds in the U.S. are infected with
bovine leukemia virus. This virus has also been linked to
breast cancer in humans.