Last winter, a potent virus with a catchy name began ravaging southern Mexico. By April, the pathogen, born in pigs, had bounced around the globe, infecting people in Asia, Europe and the U.S. The virus, spawned by genetic mutations, was something new — and it disproportionately afflicted the young. Health experts insisted on constant vigilance; the world girded for a once-in-a-century pandemic like the 1918 Spanish flu. The World Health Organization estimated that up to 3 billion people worldwide could become infected. Though it ripped through nearly every U.S. state — prompting President Obama to declare a national emergency in the fall — H1N1 wasn't as lethal as anticipated, even though its potential for devastation was abetted by a spotty vaccine-delivery system. As of mid-November, about 4,000 Americans had died from the virus, according to the Centers for Disease Control and Prevention — a grim tally, but far from the seismic event that some had feared.