The global battle to wipe out polio has come closer than ever to success, but obstacles remain. The hurdles are not only the highly infectious nature of the disease but also war, corruption, weak public health systems and failing states.
Last week added another obstacle to that list: obscene acts of violence. In Pakistan, nine unarmed vaccination workers were killed by gunfire while distributing medicine that could save the lives of countless children. Six of the victims were female, including three teenagers.
The attacks were unprecedented. In their wake the United Nations suspended a vaccination drive aimed at high-risk areas of the country and the young people who are most susceptible to the polio virus, which affects the nervous system and can lead to paralysis.
The perpetrators are unknown, and no one claimed responsibility, but they are believed to be the Pakistan Taliban. The Taliban has denied it, but the Associated Press reported that suspicion has fallen on the terrorist organization because of its past opposition to the campaign and because of claims by some extremists that the vaccine could make children sterile - not true - or that the workers are spies for the United States.
This is not true either but may be lingering blowback from the CIA's use of a doctor in Pakistan and a fake vaccination campaign in the hunt for Osama bin Laden.
Huge strides have been made toward eradicating polio since the effort was launched in 1988. Then, the disease was endemic in 125 countries; today, only in three: Pakistan, Afghanistan and Nigeria. Last year, there were 650 cases globally; this year, as of last week, 214.
In the 1980s the disease annually killed or paralyzed more than 350,000 children. The member states of the World Health Organization declared a "programmatic emergency" in May to galvanize efforts toward a final eradication, an exceedingly difficult task that has been achieved with only one other disease, smallpox.
Pakistan's progress is notable but fragile. This year there have been only 56 polio cases in the country, less than a third of the 175 last year. Yet the virus can spread rapidly and unexpectedly, especially where hygiene and sanitation are poor.
This is why vaccination is so important; if enough children are immunized, the virus can't find susceptible children to infect and dies out. By shooting the vaccine teams in Pakistan, the assailants are playing with fire.
They have not only snuffed out the lives of humanitarian workers but blasted open a pathway for the disease to spread among children not immunized. It is hard to contemplate an ideology that would deliberately put so many people at risk.
The government of Pakistan has rightly condemned the shootings, as have the United Nations and the WHO. The gunmen cannot be allowed to shut down a vital public health campaign. Perhaps the workers will need to be better protected, but the vaccination effort must go on.