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.