For the first time, India has gone a full year without a new polio case, the World Health Organization announced last week.
The last case, the only one in 2011, was of an 18-month-old girl in West Bengal State whose sudden paralysis was confirmed as polio on Jan. 13. There were 42 known cases in 2010.

Polio eradication officials described a year without new cases as a "game-changer" and a "milestone" because India was for decades one of the biggest centers of the disease.

But the country won't be certified as polio-free until it has gone three years without a new case, and there have been new cases recently in three nearby countries: Pakistan, Afghanistan and China. For every case of paralysis, there are an estimated 200 asymptomatic carriers, any of whom can spread the disease.

There have also been recent cases in several countries in Africa; in 2009, an Indian polio strain turned up in Angola, so the virus could theoretically travel in the opposite direction, to India from Africa.

India, which has spent more than $2 billion fighting polio, must keep vaccinating to suppress any possibility of an imported case's taking hold, and in rare cases the weakened live strain used as a vaccine can mutate back into a form that can paralyze and spread. The country's vaccine drives have tried to reach 175 million children twice a year.

India's health minister described his country as "excited and hopeful" but also "vigilant and alert."