Scientists confront a mystery: Why have U.S. bird flu cases been so mild? While grateful, many are perplexed by the H5N1 outbreak’s lack of severity

Scientists confront a mystery: Why have U.S. bird flu cases been so mild? While grateful, many are perplexed by the H5N1 outbreak’s lack of severity Since the outbreak of H5N1 bird flu in American dairy cattle began this spring, there have been 55 cases among people in the U.S. That’s an alarming number, but a surprising bright side is that all the infections have been mild so far. Bird flu has a reputation — well-earned, STAT’s Helen Branswell says — as a very dangerous pathogen, with a case fatality rate around 50%. So what’s happening here? Helen spoke to 21 researchers who have studied influenza, and this one in particular, for years. Nobody has definitive answers (“If you find out, please let me know!” one researcher told Helen), but plenty raised theories and questions. Could the way that the virus is being transmitted limit its severity? Do we actually have some protection to it already? With such limited information, the train of thought can even get existential: Has the virus changed? Have we?