It is thought that in Europe, the influenza virus changed in such a way to make it more virulent in the human population, resulting in increased severity of disease and increased mortality. What changes could have led to the differences in pathology observed in Camp Funston and in Europe?

These changes probably would have taken place because of antigenic shift or to a smaller extent because of antigenic drift. The antigenic shift takes place when the segments of the influenza viral genome recombine in various ways, and this leads to major modifications to the proteins present on the surface.

The individuals generally do not possess immunological memory to this novel strain of the virus, thus, the disease that they encounter can be more extreme in comparison to the infection with strains to which they possess certain immunological memory. The antigenic shift can result in influenza pandemics.