"If we don't control the outbreak now, the situation will get worse and worse," Philippe Barboza, the WHO's team lead on cholera, told reporters in Geneva.
He said fatality rates are extremely high for most of the countries for which the UN health agency has data.
"But this year, we have a factor which is even more important: the direct impact of climate change, with a succession of major droughts, unprecedented floods in certain parts of the world, and cyclones which have amplified most of these epidemics," he said.
Barboza said that while there had been big epidemics in certain countries before, they had not happened simultaneously, as now.
The World Health Organization is responding to cholera outbreaks in 29 countries, including Haiti, which has more than 1,200 confirmed cases, more than 14,000 suspected cases and more than 280 reported deaths.
This week, Haiti received almost 1.2 million doses of oral cholera vaccines.
But the WHO said that vaccine stockpiles were extremely low -- and that manufacturers were not enthusiastic about producing a vaccine chiefly aimed at some of the poorest countries in the world.
Although cholera can kill within hours, it can be treated with simple oral rehydration, and antibiotics for more severe cases.
But many people lack timely access to such treatment.
Outbreaks can be prevented by ensuring access to clean water and improving surveillance.
"It is not acceptable in the 21st century to have people dying of a disease which is very well-known and very easy to treat," said Barboza.
Around 36 million cholera vaccine doses were produced this year.
Barboza said that making these doses was not very attractive to manufacturers as it is "a vaccine for poor countries".
But he insisted that the mortality rate could be reduced by prioritizing timely access to medical aid.
"The fight against cholera is not lost. We can win it," he said.