Why Ebola is so dangerous

Health care workers are among those most at risk of catching Ebola

Ebola crisis

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 struggle to control the virus.

What is Ebola? Ebola is a viral illness of which the initial symptoms can include a sudden fever, intense weakness, muscle pain and a sore throat, according to the World Health Organization (WHO). And that is just the beginning: subsequent stages are vomiting, diarrhoea and - in some cases - both internal and external bleeding.

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.

This molecular model shows the parts of the Ebola virus scientists are studying in the hopes of finding drugs that will slow the spread of the disease

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.
World Health Organization guidance on Ebola
In pictures: Battling Ebola in West Africa

Where does it strike? Ebola outbreaks occur primarily in remote villages in Central and West Africa, near tropical rainforests, says the WHO.

Bushmeat - from animals such as bats, antelopes, porcupines and monkeys - is a prized delicacy in much of West Africa but can also be a source of Ebola

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.

From Nzerekore, a remote area of south-eastern Guinea, the virus has spread to the capital, Conakry, and neighbouring Liberia and Sierra Leone.
A Liberian man who flew to Lagos in July was quarantined on his arrival and later died of Ebola - the first case in Nigeria.
The medical charity Medecins Sans Frontieres (MSF) says the outbreak is "unprecedented" in the way the cases were scattered in multiple locations across Guinea, hundreds of kilometres apart, and says it is a "race against time" to check people who come into contact with sick people.

WHO: West Africa Ebola outbreak figures as of 27 July

• Guinea - 319 deaths, 427 cases
• Liberia - 129 deaths, 249 cases
• Sierra Leone - 224 deaths, 525 cases

The virus detective who discovered Ebola

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.

Washing hands and improving hygiene is one of the best ways to fight the virus

Carers should wear gloves and protective equipment, such as masks, and wash their hands regularly.
The WHO also warns against consuming raw bushmeat 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.