By Will Dunham

WASHINGTON, Jan 21 (Reuters) - An infusion of more than $630 million spearheaded by Microsoft co-founder Bill Gates is intended to help finish the job of eradicating the crippling disease polio, officials said on Wednesday.

The Seattle-based Bill & Melinda Gates Foundation and the Rotary International charity joined with the governments of Britain and Germany to commit the money over the next five years to support child immunization.

An international effort has cut polio cases by 99 percent in 10 years -- a drop from more than 350,000 cases in 1998 to about 1,600 in 2008.

The polio virus remains endemic in only Afghanistan, India, Nigeria and Pakistan, but imported cases from these four countries threaten other developing nations.

In a conference call with reporters, Gates declined to set a target date to achieve the goal of polio eradication "because nobody knows what it will take."

"We are in the end game," Gates said. "I'm optimistic that we will be successful. I'm personally very committed."

The Gates Foundation, created by the wealthy U.S. businessman and his wife, pledged $255 million and Rotary International $100 million. Britain will provide about $150 million and Germany about $130 million, the foundation said.

The money will support a global initiative led by the U.N. World Health Organization and other partners.

Global health leaders are hoping to make polio the next disease to be eliminated worldwide, much as smallpox was declared eradicated three decades ago.

'VERY CLOSE TO ERADICATION'

Polio is a virus that attacks the nervous system and can cause paralysis, difficulty breathing and sometimes death. It affects mainly children under the age of 5.

Mass immunization campaigns in the 20th century slowed polio's spread and eventually chased it from most nations.

Gates said there already has been a global investment of $6 billion for the polio-fighting initiative but he cited some setbacks. More children were infected with polio in 2008 than in 2007, Gates noted.

Nigeria, which has struggled to contain polio since some northern states imposed a yearlong vaccine ban in mid-2003, accounts for more than 50 percent of new cases.

Also, in September, a suicide bomber in Afghanistan attacked a marked U.N. vehicle, killing two Afghan doctors working on polio vaccination for the WHO.

"We are very close to eradication," said Adolf Kloke-Lesch of the German Ministry for Economic Cooperation and Development, urging other countries -- especially the rest of the Group of Eight industrialized nations -- to contribute.

"The political will and commitment of government and partners in the four countries still affected by polio will also be crucial to win the fight," Kloke-Lesch told reporters.

British International Development Secretary Douglas Alexander called the new infusion of money "a massive boost in the battle to rid the world of the scourge of polio." (Editing by Maggie Fox and John O'Callaghan)

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