Health workers backed by helicopters and dugout canoes raced on Tuesday to immunize children across Botswana against polio after the first case in a decade raised fears of a resurgence of the disease.

The sparsely populated country launched its first-ever compulsory immunization last month and even won a High Court order overriding objections from religious groups opposed to vaccinations.

“Botswana was polio free, but the virus moves extremely fast - just one case can mean 100,000 people exposed to the disease,” said the head of U.N. children’s agency UNICEF in Botswana, Gordon Jonathan Lewis.

In February a five-year-old child was found to have a strain of polio normally found 2,500 km (1,500 miles) away in Nigeria. It was Botswana’s first case since 1991, and experts believe the infection originated in Nigeria even though no direct contact could be proved.

This week’s operation aims to give every child in Botswana under five years old a second dose of polio vaccine by Friday.

“We have traversed the Kalahari Desert, the flooded Okavango wetlands, traveled in helicopters, boats, mokoro (dugout canoes), and four-wheel drive vehicles to get the vaccine to where it was needed,” UNICEF spokeswoman Sarah Crowe said.

Once among the poorest countries on earth, Botswana has achieved one of the highest per capital incomes in Africa thanks to massive diamond reserves, and has won international praise for using the proceeds to fund education and the war on AIDS.

Lewis said the program was a model for other countries.

“We urge governments to follow the example set by Botswana and show political and financial commitment - the government is paying the lion’s share, with UNICEF and the WHO only topping up the budget - to eradicate polio,” said UNICEF’s Lewis.