Story highlights CDC: Ebola cases in Liberia and Sierra Leone could rise to 550,000 by January

WHO: Death toll from Ebola passed 2,800 as of Monday, with 5,800-plus cases reported

Fatality rate is 71% and "current epidemiologic outlook is bleak," WHO experts say

Nigeria and Senegal have shown Ebola can be contained with rigorous efforts, it says

The number of Ebola cases in Liberia and Sierra Leone could rise to between 550,000 and 1.4 million by January if there are no "additional interventions or changes in community behavior," the Centers for Disease Control and Prevention said in a report Tuesday. The estimate was derived from a new forecasting tool developed by the CDC.

The range of estimated cases -- from 550,000 to 1.4 million -- is wide because experts suspect the current count is highly under-reported. The official death toll from Ebola in West Africa has climbed to more than 2,800 in six months, with 5,800 cases confirmed as of Monday, the World Health Organization said.

But the CDC estimates that if 70% of people with Ebola are properly cared for in medical facilities, the epidemic could decrease and eventually end.

In a press conference Tuesday, CDC Director Tom Frieden cautioned that this model is based on older data from August. The numbers are not projections, but "scenarios." The model does not take into account President Obama's announcement that the U.S. is sending troops and extra medical equipment to the area. Nor does it take into account the additional help from other countries promised.

What the model does suggests, he said, is that the current surge of help can "break the back of the epidemic" and is "exactly what's needed" to end it. He said he is now "confident the most dire predictions will not come to pass."

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Photos: The Ebola epidemic Photos: The Ebola epidemic An Ebola survivor participates in a study in Monrovia, Liberia, on June 17, 2015. The country launched a five-year study to unravel the mystery of the long-term health effects that plague survivors of the viral disease. Since the epidemic started more than a year ago in a remote village in Guinea, more than 11,000 people have died, the vast majority in three West African nations, according to the latest numbers from the World Health Organization . And that number is believed to be low, since there was widespread under-reporting of cases, according to WHO. Hide Caption 1 of 45 Photos: The Ebola epidemic Women in Monrovia celebrate after the World Health Organization declared Liberia Ebola-free on May 9, 2015. Other cases have recurred since, however. Two people in Liberia have died of the disease since the end of June, just weeks after the WHO declared the nation free of the disease. Hide Caption 2 of 45 Photos: The Ebola epidemic A man walks past an Ebola awareness painting in Monrovia on March 22, 2015. Hide Caption 3 of 45 Photos: The Ebola epidemic Soldiers from the U.S. Army's 101st Airborne Division walk across the tarmac at Campbell Army Airfield before reuniting with their families at a homecoming ceremony March 21, 2015 in Fort Campbell, Kentucky. The 162 soldiers were deployed in Liberia, where they helped fight the spread of Ebola. Hide Caption 4 of 45 Photos: The Ebola epidemic Relatives weep for a loved one who it was believed died from Ebola, at a graveyard on the outskirts of Monrovia on March 11, 2015. Hide Caption 5 of 45 Photos: The Ebola epidemic Doctors Without Borders staffer Alex Eilert Paulsen watches as mattresses and bed frames burn at the Ebola Treatment Unit in Paynesville, Liberia, on January 31, 2015. The organization reduced its number of beds from 250 to 30 as gains were made in battling the virus. Hide Caption 6 of 45 Photos: The Ebola epidemic Pauline Cafferkey, a Scottish woman diagnosed with Ebola, is put on a plane in Glasgow, Scotland, on December 30, 2014. Cafferkey, a 39-year-old nurse who volunteered in Sierra Leone, was being transported to London for treatment. Hide Caption 7 of 45 Photos: The Ebola epidemic A child who survived the Ebola virus is fed by another survivor at a treatment center on the outskirts of Freetown, Sierra Leone, on November 11, 2014. Hide Caption 8 of 45 Photos: The Ebola epidemic Health workers in Monrovia cover the body of a man suspected of dying from the Ebola virus on October 31, 2014. Hide Caption 9 of 45 Photos: The Ebola epidemic Kaci Hickox leaves her home in Fort Kent, Maine, to take a bike ride with her boyfriend on October 30, 2014. Hickox, a nurse, recently returned to the United States from West Africa, where she treated Ebola victims. State authorities wanted her to avoid public places for 21 days -- the virus' incubation period. But Hickox, who twice tested negative for Ebola, said she would defy efforts to keep her quarantined at home. Hide Caption 10 of 45 Photos: The Ebola epidemic Health officials in Nairobi, Kenya, prepare to screen passengers arriving at the Jomo Kenyatta International Airport on October 28, 2014. Hide Caption 11 of 45 Photos: The Ebola epidemic U.S. President Barack Obama hugs Ebola survivor Nina Pham in the Oval Office of the White House on October 24, 2014. Pham, one of two Dallas nurses diagnosed with the virus, was declared Ebola-free after being treated at a hospital in Bethesda, Maryland. The other nurse, Amber Vinson (not pictured), was treated in Atlanta and also declared Ebola-free. Hide Caption 12 of 45 Photos: The Ebola epidemic Health workers in Port Loko, Sierra Leone, transport the body of a person who is suspected to have died of Ebola on October 21, 2014. Hide Caption 13 of 45 Photos: The Ebola epidemic Health workers bury a body on the outskirts of Monrovia on October 20, 2014. Hide Caption 14 of 45 Photos: The Ebola epidemic Garteh Korkoryah, center, is comforted during a memorial service for her son, Thomas Eric Duncan, on October 18, 2014, in Salisbury, North Carolina. Duncan, a 42-year-old Liberian citizen, died October 8 in a Dallas hospital. He was in the country to visit his son and his son's mother, and he was the first person in the United States to be diagnosed with Ebola. Hide Caption 15 of 45 Photos: The Ebola epidemic Boys run from blowing dust as a U.S. military aircraft leaves the construction site of an Ebola treatment center in Tubmanburg, Liberia, on October 15, 2014. Hide Caption 16 of 45 Photos: The Ebola epidemic Aid workers from the Liberian Medical Renaissance League stage an Ebola awareness event October 15, 2014, in Monrovia. The group performs street dramas throughout Monrovia to educate the public on Ebola symptoms and how to handle people who are infected with the virus. Hide Caption 17 of 45 Photos: The Ebola epidemic Ebola survivors prepare to leave a Doctors Without Borders treatment center after recovering from the virus in Paynesville, Liberia, on October 12, 2014. Hide Caption 18 of 45 Photos: The Ebola epidemic A man dressed in protective clothing treats the front porch of a Dallas apartment on October 12, 2014. The apartment is home to one of the two nurses who were diagnosed with Ebola after treating Thomas Eric Duncan, a Liberian national who traveled to Dallas and later died from the virus. Hide Caption 19 of 45 Photos: The Ebola epidemic A woman crawls toward the body of her sister as a burial team takes her away for cremation October 10, 2014, in Monrovia. The sister had died from Ebola earlier in the morning while trying to walk to a treatment center, according to her relatives. Hide Caption 20 of 45 Photos: The Ebola epidemic A man digs a grave on October 7, 2014, outside an Ebola treatment center near Gbarnga, Liberia. Hide Caption 21 of 45 Photos: The Ebola epidemic A person peeks out from the Dallas apartment where Thomas Eric Duncan, the first person diagnosed with the Ebola virus in the United States, was staying on October 3, 2014. Hide Caption 22 of 45 Photos: The Ebola epidemic A girl cries as community activists approach her outside her Monrovia home on October 2, 2014, a day after her mother was taken to an Ebola ward. Hide Caption 23 of 45 Photos: The Ebola epidemic A health official uses a thermometer September 29, 2014, to screen a Ukrainian crew member on the deck of a cargo ship at the Apapa port in Lagos, Nigeria. Hide Caption 24 of 45 Photos: The Ebola epidemic Workers move a building into place as part of a new Ebola treatment center in Monrovia on September 28, 2014. Hide Caption 25 of 45 Photos: The Ebola epidemic Medics load an Ebola patient onto a plane at Sierra Leone's Freetown-Lungi International Airport on September 22, 2014. Hide Caption 26 of 45 Photos: The Ebola epidemic A few people are seen in Freetown during a three-day nationwide lockdown on September 21, 2014. In an attempt to curb the spread of the Ebola virus, people in Sierra Leone were told to stay in their homes. Hide Caption 27 of 45 Photos: The Ebola epidemic Supplies wait to be loaded onto an aircraft at New York's John F. Kennedy International Airport on September 20, 2014. It was the largest single shipment of aid to the Ebola zone to date, and it was coordinated by the Clinton Global Initiative and other U.S. aid organizations. Hide Caption 28 of 45 Photos: The Ebola epidemic A child stops on a Monrovia street September 12, 2014, to look at a man who is suspected of suffering from Ebola. Hide Caption 29 of 45 Photos: The Ebola epidemic After an Ebola case was confirmed in Senegal, people load cars with household items as they prepare to cross into Guinea from the border town of Diaobe, Senegal, on September 3, 2014. Hide Caption 30 of 45 Photos: The Ebola epidemic A health worker wearing a protective suit conducts an Ebola prevention drill at the port in Monrovia on August 29, 2014. Hide Caption 31 of 45 Photos: The Ebola epidemic A burial team from the Liberian Ministry of Health unloads bodies of Ebola victims onto a funeral pyre at a crematorium in Marshall, Liberia, on August 22, 2014. Hide Caption 32 of 45 Photos: The Ebola epidemic Dr. Kent Brantly leaves Emory University Hospital on August 21, 2014, after being declared no longer infectious from the Ebola virus. Brantly was one of two American missionaries brought to Emory for treatment of the deadly virus. Hide Caption 33 of 45 Photos: The Ebola epidemic An Ebola Task Force soldier beats a local resident while enforcing a quarantine on the West Point slum on August 20, 2014. Hide Caption 34 of 45 Photos: The Ebola epidemic Local residents gather around a very sick Saah Exco, 10, in a back alley of the West Point slum on August 19, 2014. The boy was one of the patients that was pulled out of a holding center for suspected Ebola patients after the facility was overrun and closed by a mob on August 16. A local clinic then refused to treat Saah, according to residents, because of the danger of infection. Although he was never tested for Ebola, Saah's mother and brother died in the holding center. Hide Caption 35 of 45 Photos: The Ebola epidemic Workers prepare the new Ebola treatment center on August 17, 2014. Hide Caption 36 of 45 Photos: The Ebola epidemic Liberian police depart after firing shots in the air while trying to protect an Ebola burial team in the West Point slum of Monrovia on August 16, 2014. A crowd of several hundred local residents reportedly drove away the burial team and their police escort. The mob then forced open an Ebola isolation ward and took patients out, saying the Ebola epidemic is a hoax. Hide Caption 37 of 45 Photos: The Ebola epidemic A health worker disinfects a corpse after a man died in a classroom being used as an Ebola isolation ward August 15, 2014, in Monrovia. Hide Caption 38 of 45 Photos: The Ebola epidemic Aid worker Nancy Writebol, wearing a protective suit, gets wheeled on a gurney into Emory University Hospital in Atlanta on August 5, 2014. A medical plane flew Writebol from Liberia to the United States after she and her colleague Dr. Kent Brantly were infected with the Ebola virus in the West African country. Hide Caption 39 of 45 Photos: The Ebola epidemic Members of Doctors Without Borders adjust tents in the isolation area in Kailahun on July 20, 2014. Hide Caption 40 of 45 Photos: The Ebola epidemic Boots dry in the Ebola treatment center in Kailahun on July 20, 2014. Hide Caption 41 of 45 Photos: The Ebola epidemic Dr. Jose Rovira of the World Health Organization takes a swab from a suspected Ebola victim in Pendembu, Sierra Leone, on July 18, 2014. Hide Caption 42 of 45 Photos: The Ebola epidemic Red Cross volunteers disinfect each other with chlorine after removing the body of an Ebola victim from a house in Pendembu on July 18, 2014. Hide Caption 43 of 45 Photos: The Ebola epidemic A scientist separates blood cells from plasma cells to isolate any Ebola RNA and test for the virus April 3, 2014, at the European Mobile Laboratory in Gueckedou, Guinea. Hide Caption 44 of 45 Photos: The Ebola epidemic Health specialists work March 31, 2014, at an isolation ward for patients at the facility in southern Guinea. Hide Caption 45 of 45

The cautionary tale this data does tell, he said is that there would be an "enormous cost" if help is delayed.

Gayle Smith , a special assistant to the President and senior director of the National Security Council said the model tells responders how to "bend the curve" and that the epidemic can end with a large international response. "The momentum must at least be maintained to ensure we are way out ahead of this," Smith said. And added that the international help will need to "stay on this as long as it takes."

A separate nine-month assessment published by WHO experts in The New England Journal of Medicine on Tuesday says the fatality rate of this outbreak in West Africa is 71% and that the "current epidemiologic outlook is bleak."

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It also warns that without "drastic improvements" in measures to control its spread, the number of cases and deaths from Ebola is expected to continue climbing from hundreds to thousands per week in the coming months. The cumulative number of cases could exceed 20,000 by November 2, the assessment said.

It blamed the spread not on a particularly virulent strain of the virus, rather it said this outbreak was so deadly because of a "combination of dysfunctional health systems, international indifference, high population mobility, local customs, densely populated capitals, and lack of trust in authorities after years of armed conflict."

"Perhaps most important, Ebola has reached the point where it could establish itself as an endemic infection because of a highly inadequate and late global response."

It warns that the global response must improve to meet the threat posed by this and future epidemics.

Another six-month report from WHO -- which assesses the situation in three countries at the outbreak's center, Guinea, Liberia and Sierra Leone -- is dedicated to those health workers "who, as an expression of our innately shared human compassion, risked their lives, and lost them."

In the six months since this outbreak was first formally reported in March, 337 health care workers have been infected, of whom more than 181 have died, the WHO said.

Guinea: Porous borders aid spread

The WHO's six-month report on Guinea , where the outbreak began in a remote area, details how the virus first gained a grip on a vulnerable, frightened population ill-equipped to deal with the deadly disease.

Once Ebola was identified as the killer, a national and international response swung into gear. But each time it looked like the country might be at the point of controlling the virus, new flare-ups occurred.

The WHO report concludes Ebola kept getting reintroduced "into Guinea -- with its notoriously porous borders -- from the large outbreaks in neighboring Liberia and Sierra Leone."

Controlling the virus then will not be feasible in Guinea until its stopped in neighboring countries.

Meanwhile, health workers there have come under attack as they seek to educate the local population.

"Today, one of the biggest barriers to control is violence from an impoverished, terrified and shattered population that does not understand what hit it and fights back the only way it can," the report said.

Liberia: Misery and hope

WHO's six-month report on Liberia summarizes their situation as one of "misery and despair tempered by some good reasons for hope."

The number of Liberian cases is increasing exponentially, with 113 new ones reported over a single 24-hour period on one day last week, WHO said -- a record number in any outbreak. At the same time, treatment centers are overflowing. Many patients are turned away.

The capacity to deliver other basic health services, such as care for women in childbirth, has been compromised. The report warns malaria-related deaths may soon surpass those from Ebola.

Hope can be found in the opening of a new WHO-funded, 120-bed Ebola treatment center, it said. The U.S. government is also setting up a new laboratory to speed up diagnosis. Currently there are four major Ebola treatment units in Liberia.

In remarks made on Tuesday, Liberian President Ellen Johnson Sirleaf said that her country's health care system is hurting. Of the over 1500 who have died, 85 were health care workers.

The President characterized the CDC's model for possible infections as "scary to all of us." She added, though, that she is happy with the U.S. response to the disaster. She believes it will make a "major" difference.

She predicts with that increased international effort, Liberia will see better results in a couple of weeks.

Sierra Leone: Urgent needs

The situation assessment for Sierra Leone explains how traditional burial practices helped the virus spread quickly there following the funeral of a respected local healer.

The virus has taken a devastating toll on health workers and researchers.

"Sierra Leone's most urgent needs include more treatment beds in much safer facilities, better contact training and follow-up, more personal protective equipment and body bags, and more properly protected teams to collect bodies and bury them safely," WHO said.

The deployment of international medical teams in West Africa, including from China and Cuba, may help.

Nigeria and Senegal , where the situation is judged "stable -- for the moment," are seen as success stories.

Those countries have shown conventional methods such as early detection, contact tracing, isolation and an adequate supply of personal protective equipment can contain the virus, WHO said.

"If Nigeria can control an outbreak caused by such a deadly and highly contagious virus, right from the start, any country in the world can do the same," its report said.