The World Health Organisation (WHO) has declared the Ebola outbreak in West Africa to be an international public health emergency that requires an extraordinary response to stop its spread.
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 on Friday is "a clear call for international solidarity" but acknowledged that many countries would probably not have any Ebola cases.
"Countries affected to date simply do not have the capacity to manage an outbreak of this size and complexity on their own,'' Chan said at a news conference in Geneva.
"I urge the international community to provide this support on the most urgent basis possible," she added.
The agency had convened an expert committee this week to assess the severity of the ongoing epidemic - the largest and longest in history.
The current outbreak of Ebola began in Guinea in March and has since spread to Sierra Leone and Liberia.
The WHO said the situation is likely to get worse in the coming months.
There is no licenced treatment or vaccine for Ebola and the death rate has been about 50 percent.
The impact of the WHO declaration is unclear; the declaration about polio does not yet seem to have slowed the spread of virus.