New Delhi, Feb. 14 -- In a classroom in southern Malawi, children sit in rows on the floor as a health worker moves among them administering an oral vaccine that protects against polio.

The new vaccination campaign that began on Wednesday is another reminder that the world still hasn't managed to eradicate the ancient disease, which primarily affects children and can cause paralysis, despite a concerted effort for more than 35 years by the World Health Organization and its partners.

Health officials believe they came close several times, including five years ago, when just five cases of the natural polio virus were reported globally.

But a WHO report said that there were 38 cases of the natural polio virus between January and October 2...