This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax firstname.lastname@example.org with numerous contributions by others.
Polio is a contagious viral disease which crippled tens of thousands
in the 1950s, and killed more than a thousand a year. Because it is a
mild gastrointestinal illness in young children and a serious
paralytic illness in older people, it had an unusual epidemiology,
with more cases of paralytic polio turning up in wealthy areas and as
sanitation improved. 80-90% of cases of polio are the minor illness;
the rest are paralytic poliomyelitis. In paralytic poliomyelitis, <
25% suffer permanent severe disability, about 25% have mild
disabilities, and > 50% recover with no residual paralysis. Mortality
is 1 to 4%. "Recently, a post poliomyelitis syndrome has been
described, characterized by muscle fatigue and decreased
endurance... The syndrome occurs many years after an attack of
paralytic poliomyelitis..." (Merck).