The deadly Ebola outbreak sweeping across three countries in West Africa is likely to last 12 to 18 months more, much longer than anticipated, and could infect hundreds of thousands of people before it is brought under control, say scientists mapping its spread for the federal government.

“We hope we’re wrong,” said Bryan Lewis, an epidemiologist at the Virginia Bioinformatics Institute at Virginia Tech.

Both the time the model says it will take to control the epidemic and the number of cases it forecasts far exceed estimates by the World Health Organization, which said last month that it hoped to control the outbreak within nine months and predicted 20,000 total cases by that time. The organization is sticking by its estimates, a W.H.O. spokesman said Friday.

But researchers at various universities say that at the virus’s present rate of growth, there could easily be close to 20,000 cases in one month, not in nine.

The UN Security Council will convene for an emergency meeting Thursday to address the ongoing Ebola epidemic spreading across West Africa, according to the president of the 15-member body. This will be only the second time in history the council has met to address a public health crisis…

“The situation on the ground is dire and is growing worse by the day,” Powers said.

“The trendlines in this crisis are grave, and without immediate international action we are facing the potential for a public health crisis that could claim lives on a scale far greater than current estimates, and set the countries of West Africa back a generation,” added Power. “This is a perilous crisis but one we can contain if the international community comes together to meet it head on.”

President Barack Obama declared Tuesday that the Ebola epidemic in West Africa could threaten security around the world, and he ordered 3,000 U.S. military personnel to the region in emergency aid muscle for a crisis spiraling out of control.

The question was whether the aid would be enough and was coming in time. An ominous World Health Organization forecast said that with so many people now spreading the virus, the number of Ebola cases could start doubling every three weeks…

“It’s a potential threat to global security if these countries break down,” Obama said, speaking of the hardest-hit nations of Liberia, Sierra Leone and Guinea. At least 2,400 people have died, with Liberia bearing the brunt. Nearly 5,000 people have fallen ill in those countries and Nigeria and Senegal since the disease was first recognized in March. WHO says it anticipates the figure could rise to more than 20,000, and the disease could end up costing nearly $1 billion to contain.

“The reality is that this epidemic is going to get worse before it gets better,” Obama said at the U.S. Centres for Disease Control and Prevention’s Atlanta headquarters…

The U.S. plan, a dramatic expansion of Washington’s initial response last week, won praise from the U.N. World Health Organization, aid workers and officials in West Africa. But health experts said it was still not enough to contain the epidemic, which is quickly growing and has caused local healthcare systems to buckle under the strain of fighting it…

“We don’t know where the numbers are going on this,” WHO Assistant Director-General Bruce Aylward told a news conference in Geneva, calling the crisis “unparalleled in modern times.”

The president will go beyond the 25-bed portable hospital that Pentagon officials said they would establish in Liberia, one of the three West African countries ravaged by the disease, officials said. Mr. Obama will offer help to President Ellen Johnson Sirleaf of Liberia in the construction of as many as 17 Ebola treatment centers in the region, with about 1,700 treatment beds.

Senior administration officials said Monday night that the Department of Defense would open a joint command operation in Monrovia, Liberia, to coordinate the international effort to combat the disease. The military will also provide engineers to help construct the additional treatment facilities and will send enough people to train up to 500 health care workers a week to deal with the crisis…

“We should see all of West Africa now as one big outbreak,” Dr. Osterholm said. “It’s very clear we have to deal with all the areas with Ebola. If the U.S. is not able or not going to do it, that’s all the more reason to say the rest of the world has to do it.”

The first possibility is that the Ebola virus spreads from West Africa to megacities in other regions of the developing world. This outbreak is very different from the 19 that have occurred in Africa over the past 40 years. It is much easier to control Ebola infections in isolated villages. But there has been a 300 percent increase in Africa’s population over the last four decades, much of it in large city slums. What happens when an infected person yet to become ill travels by plane to Lagos, Nairobi, Kinshasa or Mogadishu — or even Karachi, Jakarta, Mexico City or Dhaka?

The second possibility is one that virologists are loath to discuss openly but are definitely considering in private: that an Ebola virus could mutate to become transmissible through the air. You can now get Ebola only through direct contact with bodily fluids. But viruses like Ebola are notoriously sloppy in replicating, meaning the virus entering one person may be genetically different from the virus entering the next. The current Ebola virus’s hyper-evolution is unprecedented; there has been more human-to-human transmission in the past four months than most likely occurred in the last 500 to 1,000 years. Each new infection represents trillions of throws of the genetic dice.

If certain mutations occurred, it would mean that just breathing would put one at risk of contracting Ebola. Infections could spread quickly to every part of the globe, as the H1N1 influenza virus did in 2009, after its birth in Mexico.

But interviews with several infectious diseases experts reveal that whereas such a mutation — or more likely series of mutations — might physically be possible, it’s highly unlikely. In fact, there’s almost no historical precedent for any virus to change its basic mode of transmission so radically. “We have so many problems with Ebola, let’s not make another one that, of course, is theoretically possible but is pretty way down on the list of likely issues,” says infectious diseases expert William Schaffner of Vanderbilt University…

Here is what it would take for it to become a real airborne risk: First off, a substantial amount of Ebola virus would need to start replicating in cells that reside in the throat, the bronchial tubes and possibly in the lungs. Second, the airborne method would have to be so much more efficient than the current extremely efficient means of transmission that it would overcome any genetic costs to the virus stemming from the mutation itself…

Even viruses that are well adapted to attacking the respiratory system often have a hard time getting transmitted through the airways. Consider the experience so far with avian flu, which is easily transmitted through the air in birds but hasn’t yet mutated to become easily spreadable in that fashion among people.

In one study, pigs infected with Ebola somehow gave it to monkeys housed in the same room without touching them. But there may be other explanations for that study. A previous “Goats and Soda” blog post noted: “The lead author has pointed out it’s possible the monkeys caught the virus when droplets from the pigs splashed into their cage during cleaning.”…

“I’d say it’s highly unlikely that Ebola virus would be able to do that,” says Vincent Racaniello, a virologist at Columbia University in New York. He says in people, this virus has always been transmitted through contact with the bodily fluids of an infected person.

“I don’t know of any human virus that has switched from being transmitted by contact to being transmitted by aerosol in as long as we’ve been studying viruses, you know just over 100 years,” he says.

Think of HIV, think of hepatitis C. There have been millions of infections. These viruses still only spread through body fluids. Why should Ebola be different?

Treating Ebola patients, he said, “is not like caring for other patients. It is grueling work. The personal protective equipment we wore … becomes excruciatingly hot, with temperatures inside the suit reaching up to 115 degrees. It cannot be worn for more than an hour and a half.”

Brantly’s hospital, the ELWA Hospital in Monrovia, was the capital’s only Ebola treatment center when the disease broke out and was quickly overwhelmed. In the month and a half he was treating Ebola cases there, he told the panel, there was only one survivor…

The laboratory his hospital relied on to confirm the presence of Ebola in patients “was 45 minutes away and inadequately staffed,” he said. “A patient would arrive at our center in the afternoon and their blood specimen would not be collected until the following morning. We would receive results later that night at the earliest. Turnaround time to positively identify Ebola cases was anywhere from 12 to 36 hours after blood was drawn”—a potentially life-threatening delay. Patients languished in the isolation unit while waiting for diagnosis, potentially infecting others while they waited…

Brantly said that he was one of the luckiest ones, receiving “the best care possible in Liberia … and world-class treatment” in the United States. But, he warned, the outbreak is “a fire straight from the pit of hell. We cannot fool ourselves into thinking that the vast moat of the Atlantic Ocean will keep the flames away from our shores.”

“We can’t dawdle on this one.”