Bill Gates and other donors are giving more than $630 million US to the international effort to eradicate polio, after the disease spiked and spread to seven countries that were previously polio-free.

Gates announced Wednesday that his foundation, along with the charity group Rotary International and the British and German governments, would donate the money in the next few years.

Despite the new injection of funds, the campaign still needs $340 million US to get through 2010. The World Health Organization estimates about another $2 billion US would be needed until 2013, when it hopes that polio will have been wiped out.

"I'm very optimistic that we will succeed [in eradicating polio]," said Gates, the co-chair of the Bill & Melinda Gates Foundation, which donated $255 million US.

Polio is a water-borne disease that can cause paralysis and sometimes death. It mostly strikes children under five.

Polio has dropped by more than 99 per cent since WHO and partners launched their initiative to eradicate the disease in 1988. But the numbers of cases — fewer than 2,000 annually — have remained at a virtual standstill since 2000.

Two deadlines to get rid of polio by 2000 and 2005 were missed. No official new target date has been set, though Dr. Bruce Aylward, WHO's director of polio eradication, said that many countries were hoping to certify that polio was gone by 2013.

Polio remains endemic in Afghanistan, India, Nigeria and Pakistan. In Nigeria, low vaccination rates have allowed the virus to surge, while the war on terror makes reaching every child difficult in Afghanistan and Pakistan. Despite continued vaccination efforts, the virus remains stubbornly entrenched in India.

Last year, cases were also reported in 11 other African countries as well as Burma and Nepal.