The outbreak of the extraordinarily lethal Ebola virus has worsened in West Africa, with the contagion showing no sign of coming under control, prompting the Centers for Disease Control and Prevention on Thursday to warn Americans to avoid nonessential travel to Guinea, Liberia and Sierra Leone.

In addition to raising the health threat level to 3, the agency’s highest level, the CDC said it is sending 50 infectious-disease experts to the affected region and will assist airports in Africa in screening outbound passengers. The CDC had already alerted U.S. hospitals and doctors to be on the lookout for signs of Ebola fever and to question patients about their recent travel history and contacts.

“This is a tragic, painful, dreadful, merciless virus. It’s the largest, most complex outbreak that we know of in history,” said CDC director Tom Frieden in a news briefing Thursday.

A patient who was infected with the virus in Africa is expected to be treated at Emory University Hospital in Atlanta within several days, the university said in a statement Thursday. The patient, his or her identity undisclosed, will be treated at a special containment unit set up in collaboration with the CDC to treat people exposed to serious infectious diseases. A spokeswoman said she did not know when the patient will arrive or who the patient is. CNN, citing an unnamed source, reported that a plane left Cartersville, Ga., Thursday evening to evacuate two U.S. charity workers in Liberia infected with Ebola.

It would be the first time a patient infected with Ebola has been treated in the United States, according to a CDC spokeswoman.

“Every precaution is being taken to move those infected safely and securely, to provide critical care en route and to maintain strict isolation upon arrival in the United States,” said the CDC spokeswoman, Barbara Reynolds. “The safety and security of U.S. citizens is our highest priority. These are U.S. citizens who are returning to the United States for medical care. They are being returned under strict medical protocols for infectious diseases.”

Frieden warned that this outbreak will take at least three to six months to suppress, under the best of circumstances. There is no cure or treatment for the disease, in which the virus replicates rapidly throughout the body, causing multiple organ failure and, typically, death in a matter of days. In the latest outbreak, about 6 of 10 people infected have died.

Vaccine research is progressing, and human trials of a possible vaccine could begin as soon as September, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Thursday. The CDC’s Frieden said a vaccine is at least a year away.

That leaves officials with low-tech tools for battling the virus as it rages across West Africa and threatens to spread to other continents. Doctors and disease detectives need to find victims and quarantine them. They have to track down people with whom they’ve been in contact. Caregivers have to be meticulous in wearing formidable protective gear. Family members must be instructed on how to handle the bodies of the deceased.

That’s been the formula for stopping all previous outbreaks in Africa, with the hot virus essentially burning itself out.

“Most of those little outbreaks of varying sizes would be contained because they were in rural areas, where health workers could isolate the people and ultimately get the villages to stop doing the things that propagated it,” Fauci said.

But this outbreak is trickier, he said, because it covers a larger region and several countries with porous borders and weak health-care systems. At the same time, Fauci and other health officials say that Americans should not fear that the epidemic will take hold in the United States. That’s because Ebola, deadly as it is, is contagious only when a patient is sick with symptoms of the disease. The virus is spread by direct contact with bodily fluids after the symptoms appear.

A person who becomes infected will not show symptoms for five to 10 days, sometimes as long as three weeks. In that incubation period, the person isn’t contagious. The symptoms, when they come on, are severe, and U.S. officials are confident that they can isolate any patients should Ebola manage to spread here.

“Ebola poses little risk to the U.S. general population,” Frieden said.

“There is certainly a possibility that someone might get on a plane who is infected in Sierra Leone or Liberia and come to the United States. But the chance of it being spread here the way you are seeing there is extraordinarily low, to the point that the CDC and me and other officials feel confident that there’s not going to be an outbreak here,” Fauci said.

The World Health Organization is not recommending any travel restrictions or closure of borders at this time. The International Air Transport Association said it will follow WHO’s lead on travel restrictions.

“What is key about this disease is that if people aren’t showing symptoms, they’re not contagious,” said Jason Sinclair, a spokesman for IATA.

What sets Ebola apart from other viruses is its lethality. Even the deadliest strain of influenza, for example, kills no more than 1 percent of patients. But some areas of Africa with poor health care have been known to have a 90 percent mortality rate from Ebola during previous outbreaks.

Since the first report of the deadly virus surfaced in March, there have been 1,323 cases reported and 729 deaths, according to the WHO.

One death attributed to Ebola has been reported in Nigeria. Patrick Sawyer, 40, a Liberia-born American citizen who worked for the Liberian Finance Ministry, fell ill while traveling from Liberia to Lagos, Nigeria, with stops in Ghana and Togo, according to the Associated Press. He died in Lagos on July 25. He reportedly had been caring for his sister, who had died of Ebola several weeks earlier.

Governments across the globe, from Washington to Europe to Hong Kong, are gauging the appropriate response. White House deputy press secretary Eric Schultz said President Obama had been briefed about the Ebola outbreak. He said the Ebola crisis won’t affect next week’s summit of African leaders being held in Washington.

“I would tell you that we’re working closely with regional governments to stem the spread of the virus. We have no plans to change any elements of the U.S.-Africa Leaders Summit, as we believe all air travel continues to be safe here,” Schultz said.

About 10,000 people arrive in the United States from the affected region of Africa every three to four months, Frieden said. The CDC has protocols in place to protect against further spread of the disease. These include notification to CDC of ill passengers on a plane before arrival, investigation of ill travelers and, if necessary, quarantine. CDC also provides guidance to airlines for managing ill passengers and crew and for disinfecting aircraft.

The CDC already has a dozen staffers working in West Africa. They are operating in regions that have been torn by civil war. Frieden said one CDC group was driven away after a hostile reception and retreated to a neighboring country, though he did not give details. There have been a couple of dozen outbreaks of Ebola previous to this one, with the first in 1976. This outbreak began in Guinea in March.

Two American aid workers in Monrovia, Liberia, have contracted the virus and are in grave condition, according to a statement released by the Christian aid group Samaritan’s Purse. The group said that Kent Brantly, a physician, was offered an experimental serum sent to the country Wednesday, but that there was only enough for one person, so he asked that it be given to Nancy Writebol, a missionary worker with the Charlotte-based missionary organization SIM. However, a SIM spokesperson said both aid workers were given the serum.

“There are efforts to bring her back,” said Palmer Holt, a spokesman for SIM. “We’re feeling optimistic about the process of relocating non-essential personnel and the two patients.”

The group is evacuating some workers from Liberia, but medical staff members have been left behind to treat patients. The outbreak has prompted the Peace Corps to temporarily remove its 102 volunteers in Guinea, 108 in Liberia and 130 in Sierra Leone.

Sierra Leone has declared a state of emergency and is mobilizing police and the military to quarantine the epicenters of the disease. Sierra Leone’s top Ebola doctor fell victim to the virus and died.

“It is very worrying to hear any reports of international agencies pulling out at the time when there is an absolute need for additional medical experts and health workers to get control of this outbreak,” said Jason Cone, communications director at Doctors Without Borders. The organization said it has 552 staffers on the ground in the three affected countries and it desperately needs more resources to battle the contagion.

Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, noted that Africa is a different place than it was 25 years ago. Africans travel much more often now, and this mobility boosts the chances for Ebola to spread.

The CDC’s Frieden expressed confidence that officials will get the situation under control eventually.

“Though it will not be quick, and it will not be easy, we do know how to stop Ebola,” Frieden said. “This is a marathon, not a sprint.”

Brady Dennis contributed to this report.