In many traditional societies, most children contract mild infections of the polio virus as babies, and develop immunity to it; but if they are isolated from it by hygiene measures and do not first come into contact with it until they are a little older, it can cause severe paralysis or even death.
在許多傳統社會中，嬰兒時期若輕微感染小兒麻痺病毒，便能產生免疫能力；若因衛生習慣的隔離，等到年紀較大時才初次接觸病毒，往往會發生嚴重的麻痺症，甚至死亡。