Conventional vaccines packed with inert versions of a flu strain give your immune system the chance to develop antibodies. These identify that strain's particular version of hemagglutinin, a lollipop-shaped protein on its surface, so your body knows what to kill if infected. Every strain's hemagglutinin has a slightly different head, so Robert Liddington, a biologist at the Burnham Institute for Medical Research in California, went after its stalk. "The lollipop head changes quite easily from generation to generation, but the stalk remains stable," he says. "It's the flu's Achilles' heel." His team found a rare antibody that targets it, injected it into mice, and exposed them to 10 times the lethal dose of several seasonal flu strains, the H5N1 bird flu and the pandemic-causing 1918 Spanish flu. The drug staved off serious infection in each case, even if administered 72 hours after the initial exposure.