ADDIS ABABA, Ethiopia (AP) Two children have contracted polio near the border with Sudan, the first cases of the crippling disease in Ethiopia in four years, officials said Friday. "We had been expecting this because the border with Sudan is open. ... We think the virus migrated from Sudan," said Dr. Gebre Senbet, a polio expert with the health ministry. She said that a medical team left the Ethiopian capital of Addis Ababa Thursday for the border region to investigate the situation. "This is serious for us ... We must strengthen our surveillance in all border areas," Gebre said. She was speaking on the day United Nations agencies launched a massive immunization drive across Africa to eradicate the disease. The effort, spanning 22 countries and reaching 100 million children, is the first in a series of 2005 campaigns to stamp out polio in Africa, which saw a fierce resurgence last year, endangering global eradication efforts, said a statement by the U.N. children's agency. Polio is a waterborne disease that usually infects young children, attacking the nervous system and causing paralysis, muscular atrophy, deformation and sometimes death.