The Ebola outbreak in West Africa is the world's deadliest to date. More than 670 people have died as health officials in Guinea,Liberia and Sierra Leone work to struggle the virus.
The disease infects humans through close contact with infected animals, including chimpanzees, fruit bats and forest antelope.
It then spreads between humans by direct contact with infected blood, bodily fluids or organs, or indirectly through contact with contaminated environments. Even funerals of Ebola victims can be a risk, if mourners have direct contact with the body of the deceased.
The incubation period can last from two days to three weeks, and diagnosis is difficult. The human disease has so far been mostly limited to Africa, although one strain has cropped up in the Philippines.
Healthcare workers are at risk if they treat patients without taking the right precautions to avoid infection.
People are infectious as long as their blood and secretions contain the virus - in some cases, up to seven weeks after they recover.
It was first discovered in the Democratic Republic of Congo in 1976 since when it has affected countries further east, including Uganda and Sudan. This outbreak is unusual because it started in Guinea, which has never before been affected, and is spreading to urban areas.
What precautions should I take?
Avoid contact with Ebola patients and their bodily fluids, the WHO advises. Do not touch anything - such as shared towels - which could have become contaminated in a public place.
Carers should wear gloves and protective equipment, such as masks, and wash their hands regularly.
The WHO also warns against consuming raw bush-meat and any contact with infected bats or monkeys and apes. Fruit bats in particular are considered a delicacy in the area of Guinea where the outbreak started.
In March, Liberia's health minister advised people to stop having sex, in addition to existing advice not to shake hands or kiss. A BBC reporter in the Liberian capital Monrovia says that public awareness campaigns around Ebola have been stepped up following the death in July of renowned Liberian doctor Samuel Brisbane.
Liberia has now closed most of its border crossings and communities hit by an Ebola outbreak face quarantine to try to halt the spread of the virus.
Nigeria's main airliner Arik Air has suspended flights to Liberia and Sierra Leone and more stringent screening is being put in place at some airports. When the outbreak first began, Senegal closed its border with Guinea.
Fighting the fear and stigmatisation surrounding Ebola is one of the greatest challenges health workers face.
What can be done if I catch it?
You must keep yourself isolated and seek professional help. Patients have a better chance of survival if they receive early treatment.
There are no vaccines, though some are being tested, along with new drug therapies.
Patients frequently become dehydrated. They should drink solutions containing electrolytes or receive intravenous fluids.
MSF says this outbreak comes from the deadliest and most aggressive strain of the virus, which kills more than 90% of patients.
Other strains are less virulent and have a survival rate of up to 75%.
However, it is not known which factors allow some people to recover while most succumb.