NEW : 13 dead from a total of 60 cases of bird flu in China , officials said . The virus is normally found in birds . H7N9 was never known to infected people until last month . The new variation of bird flu has genetic characteristics that make it well-adapted to infect humans , the New England Journal of Medicine said . Cases of the virus have now spread to central and northern China .
NEW : 13 dead from a total of 60 cases of bird flu in China , officials said . The virus is normally found in birds . H7N9 was never known to infected people until last month . Cases of the virus have now spread to central and northern China .