Which part of the H1N1 virus must have changed in order for viruses to start infecting humans cell?

For the H1N1 virus to start infecting human cells, changes in the viral hemagglutinin (HA) protein were crucial. This protein is responsible for binding to sialic acid receptors on the surface of host cells. Mutations in HA allowed the virus to effectively attach to and enter human respiratory cells, facilitating cross-species transmission and infection. Additionally, adaptations in other viral proteins may also contribute to enhanced virulence and transmissibility in humans.