Mutants viruses have been selected in vitro that are able to persistently infect non-neural cells. A single determinant can confer a persistent phenotype to a lytic strain in HEp-2 cells, but the presence of several mutations have a synergistic effect. These determinants are localized on the surface and inside the capsid, influencing interactions of the virus with its receptor. The molecular and cellular mechanisms of this persistent infection and its possible effects on the host are being studied in vivo. Here is my shot at simplifying what I consider to be the relevant material above.
Viruses mutate into different types during an infection.
The mutants are varied because differerent genes are changed. Cells which would normally be destroyed by the virus are not because of this. And transgenic mice have been developed and employed to serve as model organisms for a polio infection. The French at the Pasteur Institute thought this was important enough to put on their Web page along with a plethora of other material on infectious disease. Here in the States the National Institutes of Health are planning human research and needs volunteers.