She added that the world's "collective health security" depends on curbing the spread of the killer virus in West Africa, even as she acknowledged that many countries would probably not have any Ebola cases.
The Nigerian government declared containing the Ebola virus in Africa's most populous country a national emergency Friday, after two Ebola patients died and the health ministry said seven other cases were confirmed. President Goodluck Jonathan approved spending $11.7 million to fight the disease and urged schools to extend a current holiday to give experts more time to assess the Ebola threat.
Since Ebola was first identified in 1976, there have been more than 20 outbreaks in central and eastern Africa; this is the first to affect West Africa. The virus causes symptoms including fever, vomiting, muscle pain and bleeding. It is spread by direct contact with bodily fluids like blood, sweat, urine, saliva and diarrhea.
The U.N. agency convened an expert committee this week to assess the severity of the Ebola epidemic. WHO declared similar emergencies for the swine flu pandemic in 2009 and for polio in May.
The impact of WHO's declaration Friday is unclear; its similar declaration about polio doesn't yet seem to have slowed the spread of the paralytic virus.
"Statements won't save lives," said Bart Janssens, director of operations for the Doctors Without Borders charity group. "For weeks, (we) have been repeating that a massive medical, epidemiological and public health response is desperately needed. ... Lives are being lost because the response is too slow."
"I don't know what the advantage is of declaring an international emergency," added 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. "This could bring in more foreign aid but we don't know that yet."
Earlier this week, the World Bank pledged up to $200 million in emergency funding to help the countries affected by Ebola and strengthen public health systems across West Africa.
On Friday, the European Union said it would chip in an additional 8 million euros ($10.7 million) to Ebola efforts and send a second mobile lab to help with diagnostics. USAID also announced it would invest an extra $12.45 million to support the fight against Ebola.
In the United States, the Centers for Disease Control and Prevention has already warned Americans against traveling to West Africa due to the Ebola outbreak. The agency also put U.S. hospitals on alert for symptoms so they can spot potential cases.
Two Americans infected with Ebola recently received a drug never before tested in people.
The American doctor infected with Ebola, Kent Brantly, said in a statement Friday he's getting stronger every day. He and another aid worker, Nancy Writebol, are being treated in an isolation unit at Emory University Hospital in Atlanta.
Writebol's husband, David, who remains in Liberia, told reporters Friday that his wife also appears to be improving.
Next week, WHO will hold another meeting to discuss whether it's ethical to use experimental Ebola treatments in the current outbreak. There is no licensed drug or treatment for Ebola and no evidence in people that the experimental treatments work.
Other experts hoped the WHO declaration would mean that send more health workers are sent to West Africa.
"The situation is very critical and different from what we've seen before," said Heinz Feldmann, chief of virology at the U.S. 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."
WHO did not recommend any travel or trade bans Friday but said people who had close contact with Ebola patients should not travel internationally. For countries with Ebola, WHO issued various recommendations, including exit screening at international airports and border crossings to spot potential cases. It also discouraged mass gatherings.
WHO said countries without Ebola should heighten their surveillance and treat any suspected cases as a health emergency.
This week, two of the worst-hit Ebola countries Liberia and Sierra Leone brought in troops to enforce quarantines and stop people infected with the disease from traveling. Liberian authorities said no one with a fever would be allowed in or out of the country and warned some civil liberties could be suspended if needed to bring the killer virus under control.
The disease spread from Liberia to Nigeria when a man apparently sick with Ebola boarded a plane, according to the Nigerian government. Nigerian authorities say the man, who later died, was not placed into isolation for at least 24 hours after he was hospitalized. A nurse who treated him has since died from Ebola and authorities are monitoring seven other cases among people who had contact with him.
Chan said while extraordinary measures might be necessary to contain the Ebola outbreak, it was important to recognize civil rights.
"We need to respect the dignity of people and inform them why these measures are being taken," she said.