Indonesia is launching its largest immunisation campaign against polio, which is expected to reach 24 million children under the age of five.
There are fears the virus could spread to other Asian countries
The country was free of the disease for 10 years - but in March 2005 a 20-month-old boy in Java was infected.
Since then more than 200 polio cases have been reported in the country.
Officials believe the outbreak can be traced to Nigeria, where vaccinations were suspended in 2003 after radical clerics said they were a US plot.
The World Health Organization warns the virus could now spread to countries like Malaysia, China and the Philippines.
More than 750,000 health care workers and volunteers are involved in the initiative stretching across 6,000 islands in Indonesia.
The BBC's health reporter, Ania Lichtarowicz, says that if the virus was to spread to other countries, this would be another setback for the global polio eradication initiative.
Since 2003, 18 previously polio-free countries have been re-infected with the virus.
The World Health Organization's representative in Indonesia, Geor Peterson, told the BBC he was optimistic that the effort would be successful.
Two more vaccination campaigns are planned in September and November, by which point the world will know if the target of stopping transmission by the end of this year will be met.