Antigenic shift

[2] Affected species include other mammals and birds, giving influenza A the opportunity for a major reorganization of surface antigens.

[3] In the 1940s, Maurice Hilleman discovered antigenic shift, which is important for the emergence of new viral pathogens as it is a pathway that viruses may follow to enter a new niche.

When two different strains of influenza infect the same cell simultaneously, their protein capsids and lipid envelopes are removed, exposing their RNA, which is then transcribed to mRNA.

Until recently, such combinations were believed to have caused the infamous Spanish flu outbreak of 1918 which killed 40~100 million people worldwide.

However, more recent research suggests the 1918 pandemic was caused by the antigenic drift of a fully avian virus to a form that could infect humans efficiently.