It began with a just a handful of cases in Guinea in March. But it spread quickly to two other countries and is now the deadliest outbreak of Ebola virus on record.
At least 1,201 people in Guinea, Sierra Leone and Liberia have been infected by what is believed to be Ebola virus since its symptoms were first observed in March, according to the World Health Organization. 672 of them have died. That's a 55% percent mortality rate.
The WHO says "drastic action is needed" to contain Ebola, which has spread from rural areas to cities in West Africa and sparked fears that the killer virus could spread to other continents.
What's the latest?
Until recently the Ebola outbreak had been contained to three West African countries: Guinea, where it began, Liberia and Sierra Leone.
But this week a Liberian government official who had contracted the virus died in isolation at a hospital in Lagos, Nigeria.
Patrick Sawyer, a naturalized American citizen who worked in Liberia, had stopped in Nigeria for a conference but died before he could board a plane home to the U.S. He's the first American to die in the latest outbreak, though several other U.S. aid workers in Liberia have also contracted Ebola and are being treated.
Why was someone infected with Ebola allowed on a plane?
It is unknown whether Sawyer was displaying symptoms of Ebola before he flew from Monrovia, Liberia's capital, to Ghana and then to Togo to switch planes to fly to Lagos. His Minnesota-based widow, Decontee Sawyer, told CNN that he had cared for his Ebola-stricken sister in Liberia, though she said he didn't know at the time that she had the virus. When he arrived in Nigeria he told officials that he had no direct contact with anyone who had Ebola.
But since it takes between 2 and 21 days before someone infected with Ebola begins to show symptoms, there's little health officials can do to stop an infected but non-symptomatic person from flying to another country, said CNN's Chief Medical Correspondent Sanjay Gupta.
"When I left Conakry (Guinea's capital), they took my temperature at the airport and asked me to fill out a questionnaire, and that was really about it," said Gupta. "If I had been exposed for whatever reason and it was 21 days later before I got sick, there was nothing that would have prevented me from getting on that plane.
"I think it's going to happen at some point. Just from observing the whole process it's almost impossible from happening," Gupta said, before adding that he believes the virus would stand little chance of spreading very far in a developed country.
How easily could Ebola spread on a plane?
Serious viruses like Ebola may be just "a plane ride away" from reaching the developed world, according to Marty Cetron of the Centers for Disease Control and Prevention (CDC), but it is unlikely the virus would spread on a plane unless a passenger were to come into contact with a sick person's bodily fluids.
The CDC has sent guidance to American air carriers on how to identify and deal with passengers displaying Ebola symptoms -- and how they should disinfect aircraft after an infected passenger leaves a plane.
"Airline carriers, crew members, and airports can be very important partners in that front line," Cetron told CNN. "Being educated, knowing the symptoms, recognizing what to do, having a response protocol, knowing who to call -- those are really really important parts of the global containment strategies to deal with threats like this."
Health officials are also trying to track down everyone who was on any of the three flights Patrick Sawyer took to get to the heavily populated city of Lagos, in order to test them for Ebola.
What else is being done to stop the spread of the disease?
Ebola patients are being isolated by health officials in Western Africa, and those who have come into contact with them are being told to monitor their temperatures. The family of a second infected American aid worker had been living with him in Liberia, but they left before he started showing symptoms. While it's unlikely they contracted Ebola, the CDC is keeping the family on a 21-day fever watch.
The president of Liberia has closed most of the borders with neighboring countries, and the few points of entry that are still open will have Ebola testing centers. The president also placed restrictions on public gatherings and ordered hotels, restaurants and other entertainment venues to play a five-minute video on Ebola safety.
Arik Air, one of Nigeria's biggest airlines, has also suspended operations into Monrovia and Freetown, the capitals of Liberia and Sierra Leone, respectively, according to AllAfrica.com.
The UK government is also convening an emergency meeting Wednesday to discuss the threat of Ebola to Britain. There have been no cases of Ebola reported in the UK as of yet.
The CDC has also issued an alert to health workers in the U.S. to watch out for any patients who may have recently traveled to West Africa and could have contracted the virus.
Why does Ebola generate such fear?
Medecins Sans Frontieres (MSF) describes Ebola as "one of the world's most deadly diseases."
"It is a highly infectious virus that can kill up to 90% of the people who catch it, causing terror among infected communities," it says. The death rate in this outbreak has dropped to roughly 55% because of early treatment.
There is also no vaccination against it.
Of Ebola's five sub-types, the Zaire strain -- the first to be identified -- is considered the most deadly.
The WHO said preliminary tests on the Ebola virus in Guinea in March suggested that the outbreak there was this strain, although that has not been confirmed.
What is Ebola?
The Ebola virus causes viral hemorrhagic fever (VHF), which according to the U.S. Centers for Disease Control and Prevention (CDC), refers to a group of viruses that affect multiple organ systems in the body and are often accompanied by bleeding.
The virus is named after the Ebola River in the Democratic Republic of Congo (formerly Zaire), where one of the first outbreaks occurred in 1976. The same year there was another outbreak in Sudan.
The WHO says there are five different strains of the virus -- named after the areas they originated in. Three of these have been associated with large outbreaks of hemorrhagic fever in Africa.
These are the Bundibugyo -- an area of Uganda where the virus was discovered in 2007 -- Sudan and Zaire sub-types.
There has been a solitary case of Ivory Coast Ebola. This subtype was discovered when a researcher studying wild chimpanzees became ill in 1994 after an autopsy on one of the animals. The researcher recovered.
Finally, Reston Ebola is named after Reston in the U.S. state of Virginia, where this fifth strain of the Ebola virus was identified in monkeys imported from the Philippines.
The CDC says while humans have been infected with Ebola Reston, there have been no cases of human illness or death from this sub-type.
What are Ebola's symptoms?
Early symptoms of the Ebola virus include sudden onset of fever, weakness, muscle pain, headaches and a sore throat. These symptoms can appear two to 21 days after infection.
The WHO says these non-specific early symptoms can be mistaken for signs of diseases such as malaria, typhoid fever, meningitis or even the plague.
MSF says some patients may also develop a rash, red eyes, hiccups, chest pains and difficulty breathing and swallowing.
The early symptoms progress to vomiting, diarrhea, impaired kidney and liver function and sometimes internal and external bleeding.
Ebola can only be definitively confirmed by five different laboratory tests.
How is it treated?
There are no specific treatments for Ebola. MSF says patients are isolated and then supported by health care workers.
"This consists of hydrating the patient, maintaining their oxygen status and blood pressure and treating them for any complicating infections," it says.
There have been cases of healthcare workers contracting the virus from patients and the World Health Organization has issued guidance for dealing with confirmed or suspected cases of the virus.
Carers are advised to wear impermeable gowns and gloves and to wear facial protection such as goggles or a medical mask to prevent splashes to the nose, mouth and eyes.
MSF says it contained a 2012 outbreak in Uganda by placing a control area around its treatment center. An outbreak is considered over once 42 days -- double the incubation period of the disease -- have passed without any new cases.
How does it spread?
The WHO says it is believed that fruit bats may be the natural host of the Ebola virus in Africa, passing on the virus to other animals.
Humans contract Ebola through contact with the bodily fluids of infected animals.
The WHO says in Africa there have been documented cases of humans falling ill after contact with dead or ill chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines.
It says Ebola later spreads from human-to-human via contact with bodily fluids containing the virus. The virus can be spread through contact with an object contaminated with infected secretions.
Direct contact with the corpses of Ebola victims can also result in infection and the virus can be transmitted via infected semen up to seven weeks after clinical recovery.
MSF says while the virus is believed to be able to survive for some days in liquid outside an infected organism, it is fragile and chlorine disinfection, heat, direct sunlight, soaps and detergents can kill it.
MSF epidemiologist Kamiliny Kalahne says outbreaks usually spread in areas where hospitals have poor infection control and limited access to resources such as running water.
"People who become sick with it almost always know how they got sick: because they looked after someone in their family who was very sick -- who had diarrhea, vomiting and bleeding -- or because they were health staff who had a lot of contact with a sick patient," she says.
How many cases have there been?
The CDC estimates there have been more than 1,800 cases of Ebola and more than 1,300 deaths.
The last recorded outbreaks before the current one in Guinea were in 2012 -- in Uganda and Democratic Republic of Congo.
The Uganda outbreak involved a total of 24 probable and confirmed cases, and 17 deaths, according to the WHO, which declared it had ended in October 2012.
MSF said the Uganda outbreak had been the Sudan strain, while the virus found in DRC was the Bundibugyo sub-type.
According to the CDC, the most deadly outbreak was the 1976 outbreak in then Zaire, when 280 of 318 infected people died. In 2000, there were 425 cases of Ebola Sudan in Uganda, which resulted in 224 fatalities.
-- CNN's Nick Thompson contributed to this report.
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