The co-founder of Microsoft Foundation, Bill Gates, has expressed optimism that Nigeria will be able to defeat polio, despite intense challenges that make it hard to immunize children in the most vulnerable areas. Gates told VOA's Africa 54 program Wednesday that two newly reported cases of polio in Nigeria are "a disappointment" after so much success — not only in Africa, but in Pakistan and Afghanistan, the only other countries where polio is still endemic. "Our main challenge is that we have kids who the vaccinators have a tough time finding,” he said. “Reaching those kids in an insecure environment can be very complex. … obviously, we're going to need great partnerships with the governments in the region, particularly up in the northeast in Borno." But Gates said he is quite optimistic that as many kids as possible can be protected in northern Nigeria. "We're getting smarter all the time about how we find the villages, how we reach out to the traditional leaders,” he said. “We've got a great team up there. The government's working with us. We need to access kids and we need for people to understand the importance of getting all the kids to take this oral polio vaccine." The Bill and Melinda Gates Foundation — the charity started by the computer-technology magnate and his wife — has donated $3.5 billion since 2000 to eradicate polio. Nigeria has launched an emergency campaign to vaccinate millions of children against polio, after two cases were found in toddlers earlier this month — the first cases in Nigeria in two years. Officials say the 15-month-old boy and two-year-old girl likely will be paralyzed for the rest of their lives. They were stricken in northern Nigeria's Borno State — part of which is under siege by Boko Haram terrorists. The Islamic militants have not just made it extremely difficult to educate parents about the crippling disease, they also have made it dangerous and nearly impossible for health workers to immunize children. Nigerian forces are accompanying medical volunteers to guarantee their safety. Teams also are vaccinating children inside a displaced persons camp in Borno and will soon spread out to the neighboring states surrounding the Lake Chad region in Cameroon, Chad and Niger. Polio cannot be cured, and the virus struck fear in parents before Dr. Jonas Salk developed the vaccine in 1952. Kids no longer need a painful needle to get vaccinated, as the vaccine can be administered via a few drops taken orally. VOA's Africa 54 contributed to this report.