Updated — Oct. 29, 12:30 a.m. ET
As a deadly Ebola epidemic spreads across western Africa, world leaders are scrambling to find solutions. Here's what you need to know about the disease and the havoc it is wreaking:
1. Ebola is primarily ravaging the West African nations of Guinea, Liberia, Sierra Leone, Nigeria, and Senegal.
Authorities in these nations have scrambled to contain the disease. In Sierra Leone, the government quarantined a third of the entire population — about 2 million people — in an effort to fight the disease. The quarantine came after Sierra Leone had previously declared a state of emergency and deployed troops to clinics.
In Nigeria, President Goodluck Jonathan declared a state of emergency over Ebola on Aug. 8. Jonathan also approved the release of 1.9 billion naira ($11.7 million) to contain the disease through an intervention plan.
By Sept. 30, the Ebola outbreak in Nigeria appeared to be contained, according to the Centers for Disease Control. The situation in Senegal was "stable" at the end of September, the World Health Organization reported.
Liberia hasn't been so successful; President Ellen Johnson Sirleaf declared a state of emergency Aug. 6, but by the end of September experts warned that the disease had nevertheless taken the country to the brink of complete meltdown.
The CDC had a standing warning against nonessential travel to the affected countries. The U.S. Peace Corps also evacuated hundreds of its volunteers in affected countries.
A separate, unrelated outbreak of Ebola has also appeared in Congo.
2. The disease has spread to the United States, but many of the Western cases have been cured.
On Oct. 23, Dr. Craig Allen Spencer tested positive for Ebola in New York City. Spencer had been treating Ebola patients in West Africa while working with Doctors Without Borders. He returned to the U.S. on Oct. 17.
Spencer is the fourth person diagnosed with the virus while in the U.S.
The first person to test positive for Ebola in the U.S. was Thomas Duncan, who arrived in Dallas on Sept. 20. He developed symptoms several days later. Duncan is believed to have contracted the disease while traveling in Liberia.
On Oct. 8, Duncan died in a Dallas hospital after a week of treatment.
Two nurses — Nina Pham and Amber Vinson — contracted Ebola after treating Duncan. Pham was later taken to the National Institutes of Health in Bethesda and was in good condition on Oct. 23. Vinson was moved to Emory University Hospital in Atlanta. On Oct. 22, her blood tests came up negative for Ebola. She was released from the hospital on Oct. 28 after additional testing showed that she is virus-free, a hospital spokeswoman told the Associated Press.
One women was quarantined in New Jersey and tested for Ebola, but her test returned negative. A young boy who was suspected of having Ebola in New York City also tested negative.
Several other Americans contracted the disease in Africa. Missionary Nancy Writebol and Dr. Kent Brantly both returned to the U.S. after falling ill in July. They had recovered by late August.
American aid worker Richard Sacra also contracted the disease and was evacuated to the U.S. He was discharged from a Nebraska hospital in September.
Ebola patients have been evacuated to other countries as well. In August, a Spanish missionary and a British health care worker were both evacuated to their respective countries. The Spanish missionary later died from the disease.
The three Americans who have contracted and survived Ebola, from left: Richard Sacra, Nancy Writebol, and Kent Brantly.
3. By late October, the Ebola epidemic had killed 4,922 people.
Accounting for all confirmed, probable, and suspected deaths, the count has grown from 330 people in late June to 4,922 by late October.
The total number of cases also continues to rise, with 10,141 confirmed, probable, and suspected infections so far. The WHO warned that those numbers could "climb exponentially" to 20,000 cases by November. As many as 1.4 million people may have contracted the disease by January.
4. The Ebola epidemic has been growing in West Africa since last year.
WHO began reporting on the epidemic in March, and initial estimates indicated the outbreak began in early 2014. However, subsequent investigation traced the likely origins of the epidemic back even further, to a 2-year-old child named Emile Ouamouno who died in Guinea on Dec. 6, 2013. Ouamouno transmitted the disease to his sister, mother, and grandmother, all of whom later died as well.
The WHO says the outbreak is a "public health emergency of international concern."
5. This is the worst outbreak of Ebola in the history of the disease.
The current epidemic has both killed more people and spread to more countries than any previous Ebola outbreak.
"The outbreak is by far the largest ever in the nearly four-decade history of this disease," Margaret Chan, director-general of the WHO said.
Ebola is a relatively new disease; it was first identified in 1976 near the Ebola River in what was then Zaire (and today is the Democratic Republic of Congo). The first patient in 1976 was a 44-year-old man who was originally believed to have malaria. When scientists realized they were dealing with a new disease, they named it after the nearby river.
There have been other outbreaks since — including significant ones in 1995, 2000, 2003, and 2007 — but none that have claimed as many lives as the current one.
The current epidemic also has killed at least 208 health care workers, more than any previous Ebola outbreak.
The ongoing epidemic has been exacerbated by several factors: geography and distances; movement of both people and bodies; weak health care infrastructure in affected countries; health care workers who lack experience with Ebola; and communities that do not understand the disease and don't want to cooperate with health officials.
6. The disease is extraordinarily deadly and can kill most of the people who become infected.
During the first Ebola epidemic in 1976, the disease killed 88% of the people who became infected. According to the WHO, subsequent fatality rates have ranged from 25% to 90%.
At present, the fatality rate from this outbreak is just below 50%.
There are five different species of Ebola, three of which have been seen in Africa. The ongoing epidemic involves the Zaire Ebola virus, which is the most deadly subtype.
5. There is no licensed cure or vaccine for Ebola.
For-profit drug companies historically paid little attention to Ebola, but scientists are now working feverishly on ways to fight the disease.
In the United Kingdom, volunteers are already receiving a vaccine made by GlaxoSmithKline, a British drug manufacturer.
In the U.S., San Diego-based drug manufacturer Mapp Biopharmaceutical is making ZMapp, an experimental serum that was given to the American aid workers who contracted the disease in Liberia.
Canadian company Tekmira also is working on a drug called TKM-Ebola, which has been fast-tracked by the U.S. Food and Drug Administration. The U.S. Department of Defense is partially funding the development of TKM-Ebola.
For now, however, "raising awareness" is the primary way to fight the spread of the disease, according to the WHO. Treating those who have become infected involves using IVs to balance "the patient's fluids and electrolytes" among other things, according to the CDC. The CDC also emphasizes the use of preventative measures such isolating those who are infected and having health care workers use goggles, masks, and other protective clothing.
6. Ebola is a brutal disease that causes everything from nausea to bleeding from all of the body's orifices.
The first symptoms of Ebola include the sudden onset of fever, weakness, vomiting, diarrhea, and other things. As the disease progresses, it can cause kidney and liver failure. Symptoms typically start showing up between eight and ten days after exposure, though they can appear sooner or later.
Ebola can also cause bleeding from the eyes, ears, nose, mouth, and rectum. And it can produce swelling in the eyes and genitals, among other things.
7. The disease spreads via bodily fluids.
Ebola is not an airborne disease.
Instead, it spreads either from animals to humans, or from humans to humans. In either case, it spreads via bodily fluids. Most bodily fluids — blood, mucus, semen, saliva, etc. — can spread Ebola, as can objects and surfaces that are contaminated with infected secretions. Due to the way the virus spreads, health care workers are among the most susceptible groups of people.
People have also picked up Ebola after handling the bodies of those who died from the disease. This has been a problem in parts of Africa where burial rituals have put people in contact with infected bodies.
8. Ebola likely comes from bats.
Fruit bats are believed to be the natural host of Ebola, though ultimately scientists aren't completely sure where the disease originated. The first patient who got Ebola in 1976 became ill after handling monkey and antelope meat. People have also become sick after handling dead animals they found in the forest.
Fruit bats are actually a common food source in Guinea. In March, the government banned bat soup in an effort to fight the spread of the virus.
9. The epidemic is decimating social structures in the hardest-hit countries.
According to UNICEF, at least 3,700 children in Guinea, Liberia, and Sierra Leone have lost one or both parents to the disease. The brunt of the epidemic also has fallen disproportionately on women.
Making matters worse, thousands of those who survive the disease have been rejected by their families and communities, who fear infection themselves.
10. Despite the spread of the disease outside of West Africa, experts say it is unlikely to reach epidemic proportions in the U.S.
During Tuesday's news conference announcing the arrival of Ebola in the U.S., Frieden said there is "no doubt that we will control this importation or this case of Ebola so that it does not spread widely in this country."
The U.S. is also vastly more prepared to deal with Ebola than the countries where it has already killed hundreds or thousands. In addition to generally more developed health infrastructure, most hospitals in the U.S. are equipped to deal with the disease.
11. Officials say more needs to be done to stop the disease.
The epidemic has frequently been described as "spiraling out of control," and on Sept. 25, President Obama said the world had responded to the epidemic too slowly. The slow response to the outbreak has been a recurring criticism of the WHO.
In recent weeks, more aid has begun to flow to the affected region. In mid-September, the Bill and Melinda Gates Foundation pledged $50 million to fight the disease. Obama has also pledged aid, troops, and infrastructure development as part of a major international operation. In addition, the World Bank has put together a $400 million financing package for the region.
Still, the United Nations has estimated that it needs $1 billion to respond to the Ebola epidemic.