The WHO announced the Ebola outbreak - the largest and longest in history - is worrying enough to merit being declared an international health emergency.
WHO declared similar emergencies for the swine flu pandemic in 2009 and for polio in May.
WHO chief Dr Margaret Chan said the announcement is "a clear call for international solidarity" but acknowledged that many countries would probably not have any Ebola cases.
She said: "Countries affected to date simply do not have the capacity to manage an outbreak of this size and complexity on their own. I urge the international community to provide this support on the most urgent basis possible."
The agency convened an expert committee this week to assess the severity of the epidemic.
The current outbreak of Ebola began in Guinea in March and has since spread to Sierra Leone and Liberia. There is no licensed treatment or vaccine for Ebola and the death rate has been about 50%.
The impact of the WHO declaration is unclear as a similar announcement made about polio does not yet seem to have slowed the spread of virus.
During a WHO meeting last week to reconsider the status of polio, experts noted that countries had not yet fully applied the recommendations made in May, there have been more instances of international spread, and outbreaks have worsened in Pakistan and Cameroon.
In the United States, the Centres for Disease Control and Prevention(CDC) have already elevated its Ebola response to the highest level and it has 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.
Dr David Heymann, who directed WHO's response to the Sars outbreak and is now a professor at the London School of Hygiene and Tropical Medicine, said: "I don't know what the advantage is of declaring an international emergency.
"This could bring in more foreign aid but we don't know that yet."