Monday's announcement by both organizations came after nearly two decades of work around the world against polio, an infectious disease that can paralyze and sometimes kill.
"This investment is precisely the catalyst we need as we intensify the push to finish polio," Dr. Margaret Chan, director-general of the World Health Organization, said in a statement Monday.
Though polio incidence has been slashed by more than 99 percent worldwide since the eradication effort began in 1988, the virus remains entrenched in four countries: Afghanistan, India, Nigeria and Pakistan.
Two deadlines to eliminate polio have been missed: 2000 and 2005. More than US$5 billion (3.4 billion EUR) has been poured into eradicating polio, and some experts worry that unless the job is finished soon, the global community's money and patience may run out.
"They're on a heroic task, but money is not the only problem," said Dr. Donald A. Henderson, who headed WHO's smallpox eradication campaign in the 1970s. "We've got to soldier on. We need more money. Look at all we've accomplished. But how do we get to the endpoint?"
Henderson and other experts worry that major obstacles to vaccinating children will be harder to overcome than filling a funding gap.
In countries like Pakistan, Afghanistan, Somalia and Congo, where there are roving armies and weak health services, it has been extremely difficult to reach the high vaccination levels needed to eradicate polio.
And despite continued immunization in India, the vaccine does not work as well there, due to poor sanitation and the fact that children are often infected with other intestinal viruses.
Experts are also concerned about the eradication effort's use of the oral vaccine, which contains live polio virus. In rare instances - as in an outbreak identified in Nigeria that began in 2005 - the virus in the vaccine can mutate into a dangerous form capable of sparking new outbreaks.
The donation from Rotary International and the Gates Foundation, to be paid over three years, will largely be spent on immunization campaigns, surveillance, and public education.
"This amount of money can make quite a big difference," said Nicholas Grassly, of Imperial College, London, who advises WHO on polio issues. "We can build on the gains that have been made this year with this donation."
WHO reports that significant progress has been made in India and Nigeria, where 85 percent of the world's polio cases occur. Last year at this time, Nigeria had 958 polio cases. This year, only 226 were reported.
Still, the US$200 million falls short of the US$650 million (437 million EUR) that WHO says will be needed by 2009. Eradicating polio will ultimately cost another US$1 billion (6.7 million EUR) said Dr. David Heymann, WHO's top polio official.
This week, WHO will be holding a meeting of its polio advisory group in Geneva to discuss the progress made so far and what new strategies could help. But no new deadlines will be set.
"It looks like we're making great progress, but I'm not going to predict when polio transmission might be interrupted," Heymann said. "We need to keep our eyes on that goal."