On Friday, India will reach a major milestone in the battle against polio. January 13 will mark a year without a fresh case of polio in the country. Once all samples collected till January 12 have been cleared, India will be off the World Health Organisation’s list of polio-endemic countries, to be released on February 12. To be “polio free”, India will need to keep this performance up for two more years, and ensure that all samples stored in laboratories are free of the virus — this, officials said, was the bigger challenge.