"A picornavirus species is a polythetic class of phylogenetically related serotypes or strains which would normally be expected to share (i) a limited range of hosts and cellular receptors, (ii) a significant degree of compatibility in proteolytic processing, replication, encapsidation and genetic recombination, and (iii) essentially identical genome maps."

Note: for the ICTV 9th Report we have dropped the word polythetic from the definition, due to the misuse of the word in this context.

References

Knowles, N.J., Hovi, T, Hyypiä, T., King, A.M.Q., Lindberg, M., Pallansch, M.A.,  Palmenberg, A.C., Simmonds, P., Skern, T., Stanway, G., Yamashita, T. and Zell, R. (2011). Picornaviridae. In: Virus taxonomy: classification and nomenclature of viruses: Ninth Report of the International Committee on Taxonomy of Viruses. Ed: King, A.M.Q., Adams, M.J., Carstens, E.B. and Lefkowitz, E.J. Elsevier, San Diego, pp 855-880.

Stanway, G., Brown, F., Christian, P., Hovi, T., Hyypiä, T., King, A.M.Q., Knowles, N.J., Lemon, S.M., Minor, P.D., Pallansch, M.A., Palmenberg, A.C. and Skern, T. (2005). Family Picornaviridae. In: Virus Taxonomy. Eighth Report of the International Committee on Taxonomy of Viruses. Eds. Fauquet, C.M., Mayo, M.A., Maniloff, J., Desselberger, U. and Ball, L.A. Elsevier/Academic Press, London. p. 757-778.

King, A.M.Q., Brown, F., Christian, P., Hovi, T., Hyypiä, T., Knowles, N.J., Lemon, S.M., Minor, P.D., Palmenberg, A.C., Skern, T. and Stanway, G. (2000). Picornaviridae. In Virus Taxonomy. Seventh Report of the International Committee for the Taxonomy of Viruses. Eds Van Regenmortel, M.H.V., Fauquet, C.M., Bishop, D.H.L., Calisher, C.H., Carsten, E.B., Estes, M.K., Lemon, S.M., Maniloff, J., Mayo, M.A., McGeoch, D.J., Pringle, C.R. and Wickner, R.B. Academic Press, New-York, San Diego, pp. 657-673.