Scientists have known for some time that influenza is a weed in the lawn of life. Shaped very much like dandelions, new hemagglutinin (HA) proteins that decorate the surface of flu viruses crop up every year—persistent, hardy and unwanted. What’s more, the molecules show an uncanny ability to change annually, outwitting the previous year’s vaccine. That’s why we have to get a new flu shot every year, to fight off the HA molecule’s new look.