The counterattack against resurgent polio in Africa will be rapid, logistically difficult and potentially dangerous, involving millions of doses of vaccine, thousands of vaccinators and the health ministries and militaries of five countries, experts from the World Health Organization and other groups say.

Stopping the disease in Africa is crucial to the high-stakes effort to eradicate it from the world, a campaign that has been pressed for almost 30 years and has consumed billions of dollars.

After two years with no cases in Africa, experts were elated at the imminent taste of victory on the continent, considered the toughest front in the fight against infectious diseases. Those hopes were dashed this week when two new cases were discovered. Now Nigeria rejoins Pakistan and Afghanistan on the list of countries where the disease has not been completely eliminated.

To contain the virus before it spreads out of Borno State in Nigeria, where both paralyzed children were found last month, a first round of vaccinations will begin as early as next week. Five more rounds will take place over the coming months in ever-widening circles that ultimately encompass the entire Lake Chad basin in Nigeria, Chad, Niger, Cameroon and the Central African Republic.