TUESDAY, Sept. 2, 2014 (HealthDay News) -- As the Ebola outbreak continues to overwhelm health-care workers in three West African nations, medical experts from the United States and the United Nations called on Tuesday for a concerted international response to stem history's biggest outbreak of the often-fatal virus.
Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, said that without a greater global commitment in resources -- both medical and financial -- the epidemic will continue to spread and the number of new cases will continue to grow.
"There is a window of opportunity to stop the epidemic now, but that window is closing," Frieden said at a noon ET news conference. "We need action now. We know how it spreads. We know how to stop it," he said.
"The number of cases continues to increase and is now increasing rapidly," added Frieden, who returned Monday from a trip to West Africa, home to the hardest hit countries -- Guinea, Liberia and Sierra Leone.
The Ebola outbreak, with a mortality rate that can approach 90 percent, has killed more than 1,500 people in West Africa.
According to Frieden, the way to beat back the epidemic is with more resources, such as labs to diagnose the disease, doctors to treat it, specialists to coordinate the response and outreach to educate people.
He said Ebola is primarily spread in two ways. The first way, among people caring for people with the disease, whether at home or in health-care settings and hospitals. The second way: unsafe burial practices.
"This is not just a problem for West Africa, it is a problem for the world and the world needs to respond," said Frieden, suggesting that wealthy nations have a responsibility to join the fight against the disease.
Frieden noted that since March the U.S. government has committed $20 million to combat the outbreak. In addition, the WHO has asked for nations to commit $450 million to the fight, he said.
Unlike easily spread infectious diseases such as the flu or tuberculosis, Ebola is spread by direct contact with the blood, secretions, organs or other bodily fluids of infected people, or indirect contact with "environments contaminated with such fluids," according to the World Health Organization.
The international group Doctors Without Borders echoed on Tuesday Frieden's warnings about the dire nature of the Ebola outbreak and the race against time to contain it. The group said treatment centers in West Africa have been "reduced to places where people go to die alone."
In a briefing to the United Nations, Doctors Without Borders President Joanne Liu said that in West Africa, "cases and deaths continue to surge. Riots are breaking out. Isolation centers are overwhelmed. Health workers on the front lines are becoming infected and are dying in shocking numbers. Others have fled in fear, leaving people without care for even the most common illnesses. Entire health systems have crumbled."
Last week, the World Health Organization predicted that as many as 20,000 people could become infected within three months in Guinea, Liberia and Sierra Leone, Liu noted.
World Health Organization Director Margaret Chan said Tuesday that the outbreak will "get worse before it will get better." And, like the CDC's Frieden, she said that what's needed is a "larger global response. We need more from you. And we also need those countries that have not come on board," the Associated Press reported.
Meanwhile, another American doctor working in West Africa for a missionary group has become infected with the Ebola virus.
The missionary group, Serving In Mission (SIM), said Tuesday that the unidentified doctor had been treating women in the obstetrics ward of a hospital in the Liberian capital. It's not clear how the doctor contracted the virus because the ward is separate from the part of the hospital where Ebola patients are treated, the AP reported.
Last month two American health-care workers, including one with SIM, were infected with Ebola in Liberia and evacuated to Emory University Hospital in Atlanta. They recovered after aggressive treatment that included the experimental drug known as ZMapp.
The manufacturer says it has run out of supplies of the drug and it will take months to produce more, the AP said.
Also Tuesday, the U.S. National Institutes of Health (NIH) said the first human tests of an experimental Ebola vaccine are set to start this week.
The vaccine, which the NIH is developing with drug company GlaxoSmithKline, has "performed extremely well" in primates, but hasn't yet been tested in people, Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said in a statement, ABC News reported.
The phase 1 clinical trial with 20 people, ages 18 to 50, will begin this week at the NIH Clinical Center in Bethesda, Md., and is meant to determine if the vaccine is safe and whether it triggers an immune response that would protect against Ebola.
Ebola vaccine trials will also begin this month in England, Gambia and Mali, GlaxoSmithKline said.
The vaccine is different than the experimental Ebola drug ZMapp, which is meant to treat Ebola infection rather than prevent it.
For more on Ebola, visit the U.S. Centers for Disease Control and Prevention.
SOURCES: Sept. 2, 2014, news conference with Thomas Frieden, M.D., M.P.H., director, U.S. Centers for Disease Control and Prevention; Sept. 2, 2014, news release, Doctors Without Borders; Associated Press; ABC News