A surge in human infections of a deadly bird flu in China is prompting increasing concern among health officials around the world. While the human risk of these outbreaks is low at the moment, experts are calling for constant monitoring because of the large increase in cases this season, and because there are worrisome changes in the virus. U.S. officials say of all emerging influenza viruses, this particular virus poses the greatest risk of a pandemic threat if it evolves to spread

readily from human to human, according to a report released Friday. Centers for Disease Control and Prevention officials are developing a vaccine that would target a newly evolving version of the virus. China is experiencing its largest outbreak of the H7N9 bird flu strain, with at least 460 infections reported since October. About a third of people diagnosed with H7N9 have died of their infections, according to the World Health Organization. READ MORE