“Nearly 100 years after its discovery poliovirus remains one of most thoroughly studied and best understood virus models for the molecular virologist. While poliovirus has been of vital importance for our insight into picornavirus biology at the cellular and biochemical level, it is ironic to note that, due to the early success in defeating poliomyelitis in the developed world through vaccination, many of the basic aspects of poliovirus pathogenesis remain poorly understood.” source:
Around 70% of Poliomyelitis infections are symptom-free. The disease typically doesn't affect the central nervous system (CNS), but when it does (in about 1% of cases) it can lead to permanent muscle weakness that can be severely debilitating and, depending on the muscles affected, can be fatal.
The mechanisms, however, by which poliovirus spreads to the CNS are not fully described. It appears to be primarily a chance event — largely independent of the age, gender, or socioeconomic position of the individual.
“ [The dangerous form of ] Poliomyelitis can therefore be regarded as an 'accident' of the enteric infection, that is neither an obligatory stage of PV replication nor of any known benefit to the virus. The molecular mechanisms by which PV causes poliomyelitis are poorly understood. This is remarkable considering that PV is one of the most thoroughly investigated viruses of all times.”