DAKAR, Senegal -  At least $600 million is needed to fight West Africa's current Ebola outbreak, the World Health Organization announced Wednesday, as the death toll shot up by about 400 in a week to more than 1,900 people.

Experts warn Ebola could spread beyond the five West African countries that are already hit: Guinea, Liberia, Nigeria, Senegal and Sierra Leone.

The top priority is providing protective gear to health workers in the affected areas and ensuring that they receive hazard pay, said Dr. David Nabarro, who is coordinating the U.N. response to the unprecedented outbreak.

Doctors and nurses have been especially vulnerable to Ebola because they work closely with Ebola patients, whose bodily fluids spread the virus. Dr. Rick Sacra, 51, from the Boston area, is currently infected in Liberia, his missionary organization said Wednesday.

There now have been about 3,500 cases, WHO confirmed.

Echoing the urgency, American officials on Wednesday warned that Ebola is likely to spread to still more countries.

``This is not an African disease. This is a virus that is a threat to all humanity,'' Gayle Smith, special assistant to President Barack Obama and senior director at the National Security Council, told reporters during a telephone briefing.

Many on the ground have said there aren't enough protective suits for health workers, who have become infected in large numbers in this outbreak. The U.S. government is ``ramping up significantly'' donations of protective gear, said Smith.

The key to solving the outbreak will be implementing measures used in all previous outbreaks: isolating and treating the sick, monitoring their contacts for signs of disease and safely burying the dead, said Tom Kenyon, of the U.S. Centers for Disease Control and Prevention.

``There is a window of opportunity but it's closing with each and every day that we delay in getting measures in place,'' he said.

Kenyon said experimental vaccines and treatments would not be available in time to make a difference.

One such experimental drug is ZMapp, which has been given to seven people so far in this outbreak. The company has said that all of its doses are now exhausted, and it will be months before more can be made.

It is also still not clear if the drug is effective, since human trials have not yet been carried out. Two of the people who received ZMapp died, while five others survived including William Pooley, a British nurse who was discharged Wednesday from a London hospital.

Pooley, 29, contracted Ebola while working in Sierra Leone and was flown back to Britain on Aug. 24 where he was cared for in a special isolation unit at the Royal Free Hospital.

``I was very lucky in several ways: Firstly in the standard of care that I received, which is a world apart from what people are receiving in West Africa, despite various organizations' best efforts,'' Pooley told reporters. ``The other difference is that my symptoms never progressed to the worst stage of the disease.''