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 759 people in Guinea, Sierra Leone and Liberia have been infected by Ebola since its symptoms were first observed four months ago, according to the World Health Organization. 467 of them have died. That's a 61.5 percent mortality rate.
The WHO says "drastic action is needed" to contain the virus, which has spread from rural areas to cities in West Africa. It has dispatched teams of experts to the region and is holding talks this week with the health ministers from 11 countries about what to do next.
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.
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.