It’s true that the recent spread of bird flu among dairy cows is an “enormous concern,” as Jeremy Farrar, the World Health Organization’s chief scientist, described it last week. While only two people in the United States have contracted this H5N1 strain of the avian flu (one last year and one this month), wider spread could be catastrophic, given that, in past outbreaks, the disease has killed one of every two people who are infected.