In most people, the lung-infecting pathogens known as respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) trigger mild cold-like symptoms. But in infants and seniors, these viruses can cause severe pneumonia and even death. Vaccines against both viruses, however, have been difficult to design. Now, scientists have analyzed the structure and stability of a critical RSV and hMPV protein to better design vaccines that target it.