The polio virus has crippled and killed hundreds of thousands of children. Starting in 1957, the disease was finally, decisively eradicated (thank you, Jonas Salk) — except in Pakistan, Afghanistan, and Nigeria.
Those countries are plagued by sectarian Muslim violence, and unfortunately, Muslim fundies tend to be anti-vaxers. They’ve gotten it into their heads that polio-fighting programs are really Western-led campaigns to make Muslims infertile. As a result, terrorist groups have waged a long intimidation campaign against medical teams and even against families looking to get their children vaccinated. In recent years, dozens of vaccination workers have been assassinated.