DAKAR (Thomson Reuters Foundation) - Health workers have launched a drive to vaccinate more than 41 million children against polio in West Africa’s Lake Chad as they race to contain an outbreak of the disease in conflict-hit northeast Nigeria, the United Nations said on Tuesday.

Some 39,000 health workers have been deployed across Nigeria, neighboring Chad, Niger and Cameroon, and nearby Central African Republic, to deliver polio vaccines in high-risk areas, said the U.N. children’s agency (UNICEF).

Fighting between Islamist militant group Boko Haram and the Nigerian army is forcing people to flee their homes, raising fears that the virus could spread across borders, UNICEF said.

Boko Haram militants have killed about 15,000 people and displaced more than 2.6 million in a seven-year insurgency and continue to launch deadly attacks despite having been pushed out of the vast swathes of territory they controlled in 2014.

After two years in which polio appeared beaten in Africa, Nigeria reported three cases in August, casting a shadow over global eradication hopes, according to the World Health Organization.

“The re-emergence of polio after two years with no recorded cases is a huge concern in an area already in crisis,” said UNICEF regional director Manuel Fontaine. “We must not allow polio to spread.”

The polio virus, which invades the nervous system and can cause irreversible paralysis within hours, spreads rapidly among children, especially in unsanitary conditions in war-torn regions, refugee camps and areas where healthcare is limited.

Experts estimate that for every case of polio that paralyzes its victim, 200 silent infections go undetected.

Polio vaccination teams in Borno state in northeast Nigeria are also identifying children suffering from malnutrition, and have found high rates of severe acute malnutrition, UNICEF said.

Famine-like conditions in the former stronghold of Boko Haram could kill 75,000 children over the next year if they do not receive aid, the U.N. agency said last month.

“Children are dying and more young lives will be lost unless we scale up our response,” Fontaine said in a statement.

The Nigerian cases of polio are widely seen as a serious setback to efforts to eradicate the virus across the world.

In Pakistan and Afghanistan, the last two countries where polio remains endemic, 19 cases have been reported so far this year, the lowest ever annual tally. Until these latest infections, Nigeria’s previous polio case was in July 2014.

Polio experts said in April that stopping all transmission of the disease was possible within 2016, and that official, global eradication could then be declared by 2020.