James Estrin/The New York TimesAn Angus beef, left, and dairy cow in Harlemville, N.Y.The image of cows as placid, gentle creatures is a city slicker’s fantasy, judging from a article published on Friday by the Centers for Disease Control and Prevention, which reports that about 20 people a year are killed by cows in the United States. In some cases, the cows actually attack humans—ramming them, knocking them down, goring them, trampling them and kicking them in the head—resulting in fatal injur