The World Health Organization says India on Friday will have gone one year without a reported case of polio for the first time in the country's history.

WHO official Sona Bari welcomed the milestone, but said Thursday it is only the start of a process.

She said the next few weeks will be critical as WHO officials wait for the data on the last year to come in.

Bari said India will be considered polio-free if the virus is not detected in the next six weeks. She also noted if no cases are found in India for three years, all of WHO's South-East Asia region [Bangladesh, Bhutan, North Korea, India, Indonesia, Maldives, Myanmar, Nepal, Sri Lanka, Thailand and Timor-Leste] will be declared polio-free.

The last polio case in India was that of a two-year-old girl in the state of West Bengal on January 13, 2011.

India has long been considered the toughest country to stop the polio virus. This is due to various factors, including a poor health system and large migrant community, compounded by high population density and poor sanitation.

Polio usually infects children in unsanitary areas. It attacks the central nervous system, sometimes causing paralysis, muscular atrophy, deformation and death.