What You Need to Know About the Ebola Outbreak

Two American Ebola patients who became infected in West Africa are in isolation on U.S. soil as the outbreak's death toll swells to 2,400.

The deadly virus has infected nearly 4,800 people in Liberia, Guinea, Sierra Leone and Nigeria, killing about half of them, according to the latest numbers from the World Health Organization. Since March, this Ebola outbreak has claimed about 1,000 more lives than all the other Ebola outbreaks combined, according to WHO data.

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Here are 12 things you should know about the outbreak as fears continue to mount in Africa and beyond.

Dr. Rick Sacra Gets Blood From Ebola Survivor

Dr. Rick Sacra, a doctor working for missionary group SIM in Liberia, has been recovering from Ebola in Nebraska Medical Center since Sept. 5. Although Sacra didn't receive the experimental drug ZMapp that may have helped some other Ebola patients, he received a "convalescent serum" created from plasma donated by former Ebola patient Dr. Kent Brantly, officials at the Nebraska Medical Center said Sept. 10

"We were hoping to buy him some time, to give his immune system time to battle the disease," said Dr. Phillip Smith, head of the biocontainment unit at the Nebraska Medical Center in Omaha.

Sacra was working at ELWA Hospital in Monrovia, but he wasn't treating patients in its separate Ebola isolation facility. He was treating pregnant patients in the maternity ward, which means it's not clear how he became infected, according to SIM.

Two other Americans -- Brantly and SIM missionary Nancy Writebol -- were working with Ebola patients in Liberia when they became infected in late July. They were flown to Emory University Hospital for treatment and discharged on Aug. 19 and 21. They also received the experimental drug ZMapp, but doctors say it's not clear whether it helped them.

American Working for WHO Contracts Ebola

An unnamed American who was working for WHO in Sierra Leone contracted Ebola and was flown to Emory University Hospital for treatment on Sept. 9.

The patient is undergoing treatment in the same isolation unit where Brantly and Writebol were treated in August. Both have been declared virus-free and were discharged later that month.

US Donates Money, People, Ambulances

So far, the United States government has sent more than 100 specialists to West Africa to quell the Ebola outbreak and offer support, U.S. ambassador to the U.N. Samantha Power told ABC News. The U.S. has also spent $100 million and promised up to $75 million more.

The Department of Defense will also deploy a field hospital to Liberia, it announced Monday. The U.S. also announced Wednesday that it would supply five ambulances to Siera Leone, according to the Associated Press.

This news comes after Doctors Without Borders president Dr. Joanne Liu had some strong words for the United Nations on Sept. 2, urging its member states to do more to curb the outbreak than protect their own borders.

"We cannot cut off the affected countries and hope this epidemic will simply burn out," Dr. Joanne Liu, president of Doctors Without Borders, told the United Nations on Tuesday. "To put out this fire, we must run into the burning building."

"Doctors Without Borders ... has been ringing alarm bells for months, but the response has been too little, too late," Liu went on, declaring that the world was "losing" the battle with Ebola.

Doctors Without Borders is "completely" overwhelmed despite doubling its staff over the last month, she said, urging United Nations member states to deploy disaster response teams well versed in bio-hazard containment.

"Health workers on the front lines are becoming infected and are dying in shocking numbers," Liu said. "Others have fled in fear, leaving people without care for even the most common illnesses. Entire health systems have crumbled."

She said it is the U.N.'s responsibility to take action.

Nancy Writebol Doesn't Know How She Got Ebola

American Ebola survivor Nancy Writebol, 59, spoke out for the first time today after she was quietly discharged from Emory University Hospital on Aug. 19 and declared Ebola-free.

In an interview with Dr. Richard Besser, ABC News chief health and medical editor, Writebol, a missionary working in Liberia, said she initially thought she'd come down with malaria when her Ebola symptoms started in July. It was her job to "decontaminate" doctors as they left the Ebola isolation unit. She said she was the "mama bear."

Writebol said she doesn't know how she contracted the virus.

Her husband of 40 years, David, was tasked with delivering the news that she had Ebola.

"How do you tell the love of your life that they've contracted a deadly disease?" he told Besser.

Writebol was flown on Aug. 4 to Emory, where she stayed in an isolation unit for two weeks. Though she credits prayer, supportive care and an experimental drug ZMapp with her cure, she called the isolation ward a "really lonely place."

Ebola Survivor Nancy Writebol Recalls Dark Days With Deadly Disease

Ebola Cases Expected to Top 20,000

More than 3,000 Ebola cases have been reported since March, but the World Health Organization said last week that it expects cases to exceed 20,000 in the next six months.

The Ebola outbreak is already the deadliest on record, and WHO officials say the impact may be far worse than reported.

The number of known infections -- currently 3,069 -- is likely fewer than the actual number of infections because of those who hide the infected and bury the dead in secret, WHO said in a statement on Aug. 22. The number also excludes so-called "shadow zones" that are rumored to have Ebola cases that go unconfirmed because of community resistance and a lack of medical staff, the agency said.

Health officials also suspect an "invisible caseload" in Liberia because new treatment facilities are filling with previously unidentified Ebola patients as soon as they open.

Ebola Toll May 'Vastly Underestimate' Crisis

US Hospitals and Colleges Taking No Chances

Colleges will be screening students from West Africa for Ebola, according to the Associated Press. Some are testing students' temperatures and having private discussions with them about travel history.

Hospitals and state labs across the country recently have reported dozens of possible Ebola cases to the U.S. Centers for Disease Control and Prevention. Only 10 patients raised CDC's concerns enough to test their blood for the virus, and all of the results have come back negative so far, CDC officials told ABC News on Aug. 20.

Potential Ebola patients at Mount Sinai Hospital in New York, Johns Hopkins Hospital in Maryland, the University of New Mexico Hospital in Albuquerque and an undisclosed hospital in Ohio have also tested negative for Ebola over the past several weeks.

The CDC had sent a health alert to hospitals across the country urging them to ask patients about their travel history to help identify potential Ebola cases.

1 in 4 Americans Fears Ebola Outbreak, Poll Shows

About a quarter of Americans fear that they or someone in their family will come down with Ebola in the next year, according to a Harvard School of Public Health poll published Aug. 21.

Harvard and SSRS, an independent research company, conducted the poll of 1,025 adults and found that 39 percent of respondents feared a large Ebola outbreak in the United States.

According to the poll, 68 percent of Americans thought the disease could spread "easily" and 33 percent said they thought there was an available treatment for it, both highlighting a lack of understanding about Ebola in this country. In reality, the virus is only transmitted through contact with body fluids like blood and urine, and there is no cure. It's unclear whether ZMapp, the unofficial drug given to the American Ebola patients, helped or hindered their recovery, experts say.

Officials Request Exit Screenings at Airports, Seaports

The World Health Organization has requested exit screenings at international airports, seaports and land crossings in all countries affected by the Ebola outbreak.

"Any person with an illness consistent with [Ebola virus disease] should not be allowed to travel unless the travel is part of an appropriate medical evacuation," WHO said in a statement. "There should be no international travel of Ebola contacts or cases, unless the travel is part of an appropriate medical evacuation."

Ebola symptoms include fever, weakness, muscle pain and sore throat before they progress to vomiting, diarrhea and rash. Some people may also experience bleeding.

The WHO Ebola Emergency Committee advised against international travel restrictions or trade restrictions.

Governments Are Reviving the 'Cordon Sanitaire'

Officials from Guinea, Sierra Leone and Liberia have implemented a "cordon sanitaire" or sanitary barrier -- a cross-border isolation zone designed to contain people with the highest infection risk.

The tactic, used to prevent the spread of plague in medieval times, literally blocks off an area thought to contain 70 percent of the epidemic. But some experts say there's little proof that isolation zones can prevent the spread of disease.

Ebola-Stricken Countries Turning to Ancient Practice to Curb Outbreak

"It may not be sufficiently structured so it can prevent people from leaving," said Dr. William Schaffner, an infectious disease expert at Vanderbilt Medical Center in Nashville, Tennessee.

Fearful Communities Are Shunning Survivors

An estimated 49 percent of people infected in the outbreak have survived the virus, according to WHO data. But they face fear and shame from their communities.

Survivor Sulaiman Kemokai, from Sierra Leone, said people in his community are afraid to touch him even though he's been declared virus-free, the Associated Press reported.

Ebola Survivor Shunned by Boyfriend, School

The Ebola virus can only be spread through contact with bodily secretions such as blood, urine or sweat. But misinformation is rampant in areas hardest hit by the virus, health officials said.

FDA Warns Against Fake Ebola Treatments

The U.S. Food and Drug Administration is warning people to avoid fake Ebola treatments and vaccines being sold online. The agency said products claiming to protect people from the infection began popping up online after the outbreak began in March.

"There are currently no FDA-approved vaccines or drugs to prevent or treat Ebola," the agency said in a statement. "Although there are experimental Ebola vaccines and treatments under development, these investigational products are in the early stages of product development, have not yet been fully tested for safety or effectiveness, and the supply is very limited."

"There are no approved vaccines, drugs, or investigational products specifically for Ebola available for purchase on the Internet," the FDA added. "By law, dietary supplements cannot claim to prevent or cure disease."