While swine flu dominated the headlines, the threat from the far deadlier avian flu resurfaced. Infections from the H5N1 virus, which emerged as a threat to humans in 2003, had been on the decline for the past two years. This year, however, they reversed course: the World Health Organization clocked 49 human infections by November, compared with 44 in all of 2008. In January, scientists found that the virus which has killed more than half of the people infected since 2003 was growing resistant to a major class of antiviral drugs. Though it remains largely confined to birds, experts fear a genetic mutation could transform avian flu into a form easily spread among people. Another chilling, if still remote, possibility: scientists said bird flu could combine with swine flu, creating a third deadly super-virus.