This is a continuation of our studies previously published in this journal to use data analysis to explore why the reassortment of genetic segments from different host species and from different subtypes of influenza A viruses occurred frequently, which was considered the main reason for the generation of new strains. Of eleven proteins from influenza A virus, we have studied seven proteins in our previous studies. To get a full picture, 2352 polymerase basic proteins 1 from influenza A viruses were analysed. The results showed that the variations between host species/subtypes are smaller than those within host species/subtype. In combination with the results obtained from hemagglutinin, neuraminidase, nucleoprotein, matrix proteins 1 and 2, polymerase acidic protein and polymerase basic proteins 1 and 2, the results suggested that the predisposition to reassortment of genetic segments of influenza A virus from different host species and subtypes was mainly because of the small variations between the virus isolates.