For doctors struggling to eradicate polio, fighting the paralytic disease can mean vaccinating children in war-torn regions, persuading governments to pay attention, and begging donors for money.
A recent polio outbreak in Nigeria revealed another potential problem: the vaccine commonly used against it. Last week, the World Health Organization and the U.S. Centers for Disease Control reported that since 2005, 69 Nigerian children have been paralyzed by a polio virus derived from the oral vaccine. Two other cases made it to Niger.
Such cases are not unknown, but the continuing Nigerian outbreak is the biggest ever, and follows a boycott of the vaccination campaign in Africa's most populous country because of unfounded fears the vaccine was a Western plot to sterilize Muslims.
Officials now worry that the latest Nigerian outbreak traced to the vaccine could trigger another vaccine scare.
"This is the oral polio vaccine paradox," said Olen Kew, a virologist at the United States' Centers for Disease Control and Prevention. "This vaccine is the most effective tool we have against the virus, but it's like fighting fire with fire."
What's needed to stop Nigeria's outbreak is more of the same vaccine that caused it.
The oral polio vaccine contains a weakened version of polio virus. Children who have been vaccinated pass the virus into the water supply. Other children who then play in or drink that water pick up the vaccine's virus, which gives them some protection against polio.
But in very rare instances, as the virus passes through unimmunized children, it can mutate into a form that is dangerous enough to spark new outbreaks.
That happened for the first time in 2001, when 22 children were paralyzed in the Dominican Republic and Haiti. Subsequent vaccine-caused polio outbreaks have occurred in the Philippines, Madagascar, China and Indonesia.
Experts say these types of outbreaks only happen when not enough children are vaccinated. In northern Nigeria, only about 39 percent of children are fully protected against polio.
In the West, an injectable polio vaccine with inactivated virus is used, to avoid the problems with the oral vaccine. But the oral vaccine used in the developing world is much cheaper than the injectable vaccine, and can be given to children by volunteers with little training.
The latest outbreak linked to the vaccine comes "in the wake of all the other problems they've had in Nigeria," said Dr. Donald A. Henderson, who led WHO's smallpox eradication campaign in the 1970s.
In 2003, politicians in northern Nigeria canceled vaccination campaigns for nearly a year, claiming the vaccine was a Western plot to sterilize Muslims. That led to an explosion of polio, and the virus jumped to approximately two dozen countries worldwide.
Now, health officials' decision to keep quiet about the outbreak linked to the vaccine for so long — WHO and CDC have known about the Nigerian outbreak since last year — may look suspicious.
Dr. David Heymann, WHO's top polio official, said that because WHO considered the Nigerian outbreak to be an "operational" issue, it was unnecessary to share the information beyond its scientific committees.
CDC's Kew said added: "The people who are against immunization may seize on anything that could strengthen their position, even if it's scientifically untenable."
Rumors are still rife among Nigerians that the vaccine is unsafe, and several religious leaders continue to lecture on its dangers. If there is another mass vaccine boycott that unleashes the virus further, that could derail the global eradication effort for good.
"Convincing the Nigerians to take even more of this vaccine will be a tough sell," said Dr. Samuel Katz, an infectious diseases specialist at Duke University and co-inventor of the measles vaccine.
The delayed reporting of the Nigeria situation may also have delayed the scientific response. If WHO had shared information sooner, "the global research community could have started its laboratory and epidemiological studies earlier," said Dr. Isao Arita, of the Agency for Cooperation in International Health in Japan.
Scientists are still learning how the oral polio vaccine behaves, and need details when problems arise to determine how to avoid similar outbreaks in the future.
More than 10 billion doses have been given to children worldwide, and the vaccine has been credited with cutting polio incidence by more than 99 percent since 1988. Many more children are still paralyzed by the wild polio virus as compared to the virus in the vaccine.
But no vaccine is risk-free.
WHO said that changing the vaccination strategy is unnecessary. "It would be nice if we had a more stable oral polio vaccine, but that's not the way it is today," Heymann said. "We will continue working the way we have been working because we don't want children to be paralyzed anywhere."