By Jane Dreaper
BBC News health correspondent
Polio is still a major problem in Nigeria
Microsoft founder Bill Gates is joining the UK and German governments in a big initiative aimed at eradicating polio.
Huge strides have been made in pushing annual cases of the crippling disease down to 1,618 worldwide.
But polio persists in India, Pakistan, Afghanistan and Nigeria, and there are fears that if it not eradicated, cases could begin to climb again.
Today's pledges total £435m - but experts believe £250m is still needed to eradicate the disease.
Members of Rotary International are also pledging to raise £70m over the next three years.
The polio virus invades the nervous system, and can cause paralysis in hours. Sometimes, it can be fatal.
Survivors often have wasted limbs, and find it hard to get a job.
An analysis in the Lancet medical journal two years ago found eliminating polio would be cheaper than containing it, because even small drops in vaccination levels could lead to large outbreaks.
Bill Gates, who pledged £185m, warned the disease would continue to be a difficult problem for a number of years.
The mission is partly a personal one to him: his wife Melinda's aunt still wears braces, because she contracted polio when she was a girl.
In a speech to Rotary International, Mr Gates said: "The harsh mathematics of polio makes it clear: we cannot maintain a level of 1,000 to 2,000 cases a year.
"Either we eradicate polio, or we return to the days of tens of thousands of cases per year.
"That is no alternative at all.
"In some places children need as many as ten doses because they're already infected with so many other viruses that the vaccine can't do its job properly."
The UK is contributing £100m - even though the last recorded case of infection here was in 1982, and the last imported case in 1993.
Douglas Alexander, International Development Secretary, said: "In today's testing global economic climate, it is crucial we don't forget the health problems suffered by the world's poorest people."