The Ebola virus has now claimed nearly 4,000 lives during the current epidemic in West Africa, the largest outbreak since the virus was discovered nearly 40 years ago.

According to the World Health Organization, more than 8,000 people have been infected during the outbreak.

What is Ebola?

Ebola is a serious infectious illness which often proves fatal. Different strains kill between 50% and 90% of those infected.

The disease is caused by the Ebola virus, which is thought to have originated in fruit bats.

Ebola was first detected in 1976 in an outbreak near the Ebola River in what is now the Democratic Republic of Congo.

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How is the disease caught and spread?

Girls look at a Unicef poster on how to prevent the spread of EbolaGirls look at a Unicef poster on how to prevent the spread of Ebola

People are infected when they have direct contact through broken skin, or the mouth and nose, with the blood, vomit, faeces or bodily fluids of someone with Ebola.

The virus can be present in urine and semen too.

Infection may also occur through direct contact with contaminated bedding, clothing and surfaces – but only through broken skin.

The disease is not airborne, like flu. Very close direct contact with an infected person is required for the virus to be passed to another person.

It can take up anything from two to 21 days for humans with the virus to show symptoms.

People are not infectious until the symptoms develop.

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What are the symptoms?

Ebola virusesThe Ebola virus causes a range of painful and debilitating symptoms

The early symptoms are a sudden fever, muscle pain, fatigue, headache and sore throat.

This is followed by vomiting, diarrhoea, a rash and bleeding – both internal and external – which can be seen in the gums, eyes, nose and in the stools.

Patients tend to die from multiple organ failure or dehydration.

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How is it treated?

A vaccine trial in the USEbola vaccines are being tested on humans in trials in the US and the UK

There is no proven cure for Ebola.

Severely ill patients need to be rehydrated quickly using intravenous fluids. They should be isolated from other people and given intensive care by medical experts.

Potential vaccines are being tested. If the trials are successful they would be used to protect healthcare workers first.

Experimental drugs such as ZMapp have also been used, but they have not saved all patients.

Blood transfusions from survivors are also being tried as a potential therapy.

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Who is at risk?

Doctors wearing personal protective equipment outside an Ebola treatment unit in NigeriaDoctors wearing protective equipment outside an Ebola treatment unit in Nigeria

Anyone in close contact with patients with Ebola is at risk.

Healthcare workers treating patients, including doctors and nurses, are using protective clothing such as full-body suits and goggles, but hundreds have still died. They are at high risk.

Family members of patients are also at risk. In West African funerals, it is traditional for mourners to have direct contact with the body of the dead person, washing and embracing them before burial.

But the Ebola virus is still dangerous and present in the body after death. Prompt and safe burials are now being urged.

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Where is Ebola a problem?

A woman A woman suspected of having Ebola virus cries outside a hospital in Sierra Leone.

Guinea, Liberia and Sierra Leone in West Africa are the countries worst affected by this outbreak, which came to light in March 2014.

There have also been a small number of cases in Nigeria and Senegal, and one imported case in the US. There is also a confirmed case of Ebola in Spain.

All are linked to the current outbreak.

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Why is this outbreak particularly bad?

A health worker is decontaminated at a treatment centre Health workers have been brought into affected countries to help treat patients with Ebola

The virus has taken hold in major urban areas this time as opposed to remote, rural areas. In towns and cities which are heavily populated, people move around more and the disease is more easily spread.

Although the early cases were in Guinea, the virus quickly spread across borders to neighbouring Sierra Leone and Liberia, making the disease more difficult to contain.

The countries most severely affected by the disease in West Africa also have weak health systems. They are short of qualified doctors and nurses, and lack the appropriate equipment and resources to combat the virus, unlike in the UK.

As a result, this is the largest and most complex Ebola outbreak since the virus was discovered.

There have been more cases and deaths in this one than all other outbreaks combined.