It is the largest and longest outbreak ever recorded of Ebola, which has a death rate of about 50 per cent and has so far killed at least 932 people. WHO declared similar emergencies for the swine flu pandemic in 2009 and for polio in May.
The WHO chief, Dr Margaret Chan, said the announcement is "a clear call for international solidarity" although she acknowledged that many countries would probably not have any Ebola cases.
The agency had convened an expert committee this week to assess the severity of the continuing epidemic.
The current outbreak of Ebola began in Guinea in March and has since spread to Sierra Leone and Liberia, with a suspected cluster in Nigeria. There is no licensed treatment or vaccine for Ebola.
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The impact of the WHO declaration is unclear; the declaration about polio doesn't yet seem to have slowed the spread of virus.
In the United States, the Centre for Disease Control and Prevention have already elevated their Ebola response to the highest level and have recommended against travelling to West Africa. Yesterday, CDC director Dr Tom Frieden told a Congressional hearing that the current outbreak is set to sicken more people than all previous outbreaks of the disease combined.
"This could bring in more foreign aid but we don't know that yet," he said.
Other experts hoped the declaration would send more health workers to West Africa.
"The situation is very critical and different from what we've seen before," said Dr Heinz Feldmann, chief of virology at the US National Institute of Allergy and Infectious Disease. "There are so many locations with transmission popping up and we just need more people on the ground."