This process — called reassortment — has resulted in pandemic strains of flu, including the 1918 influenza pandemic and 2009’s swine flu (SN: 5/22/09). Viruses can’t swap parts willy-nilly.

This colorized electron microscope image released by the National Institute of Allergy and Infectious Diseases on March 26, 2024, shows avian influenza A H5N1 virus particles (yellow), grown in ...