Health workers have fanned out in northern and western Sudan to vaccinate more than five million children against the crippling effects of polio.
United Nations officials say fighters in the western Darfur region laid down their weapons Monday at the start of the three-day campaign.
The United Nations is working with Sudan's government and the World Health Organization to carry out the vaccinations, which will include southern Sudan and other areas in coming weeks.
Officials say the massive program was needed after the virus was detected in more than 100 people, mostly in the Sudanese capital, Khartoum.
A UNICEF spokeswoman told the Associated Press that the cases have been traced to Nigeria, where local officials in the northern state of Kano boycotted a vaccine drive in 2003.
Kano officials later agreed to resume polio immunizations, using an alternate source of vaccine.
Some information for this report provided by AP.