What is Ebola?
Ebola virus disease (EVD), formerly known as Ebola haemorrhagic fever, is described by the World Health Organisation (WHO) as "a severe, often fatal illness in humans."
It first appeared in 1976 in two simultaneous outbreaks - in Nzara, Sudan; and in Yambuku, in the Democratic Republic of Congo. The latter was in a village situated near the Ebola River, from which the disease takes its name.
It is mainly found in tropical Central and West Africa, and can have a 90 per cent mortality rate - although it is now at about 60 per cent.
How is it transmitted?
The virus is known to live in fruit bats, and normally affects people living in or near tropical rainforests.
It is introduced into the human population through close contact with the sweat, blood, secretions, organs or other bodily fluids of infected animals such as chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines found ill or dead or in the rainforest.
The virus then spreads in the community through human-to-human transmission, with infection resulting from direct contact (through broken skin or mucous membranes) and indirect contact with environments contaminated with such fluids.
The World Health Organisation (WHO) says the disease is not contagious until a person begins to show symptoms.
A big problem in West Africa is that burial ceremonies, in which mourners have direct contact with the body of the deceased person, can increase the spread of the disease because a person can transmit the virus even after death.
Men who have recovered from the disease can still transmit the virus through their semen for up to seven weeks after recovery from illness.
Ebola kills indiscriminately, but women are particularly badly affected. Because it is transmitted through bodily fluids, it spreads easily to people caring for the sick. In West Africa, it is generally women that prepare dead bodies for funeral and look after ailing members of their communities.
What does it do to your body?
Symptoms begin with fever, muscle pain and a sore throat, then rapidly escalate to vomiting, diarrhoea and internal and external bleeding.
The incubation period, that is, the time interval from infection with the virus to onset of symptoms, is from two to 21 days.
Health workers are at serious risk of contracting the disease - two American doctors have already contracted it, and a Liberian medic has died. Sierra Leone announced on July 23 Sheik Umar Khan, the doctor leading the fight against Ebola in the country, had himself contracted the disease following the deaths of several nurses at the treatment centre where he works.
Early treatment improves a patient's chances of survival.
How is it treated?
There is no vaccine or cure, and testing to confirm the virus must be done with the highest level of biohazard protection.
Severely ill patients require intensive supportive. Patients are frequently dehydrated and require oral rehydration with solutions containing electrolytes or intravenous fluids.
A significant problem with the current outbreak is families lose faith in Western medicine, which cannot yet cure the patients. They then take them home to traditional village healers, which often leads the disease to spread.
Where have there been outbreaks before?
The WHO is calling this the largest outbreak ever recorded of the disease.
But there have been sporadic outbreaks before - mainluy in Uganda, the DRC, Sudan and Gabon.
The worst previous outbreak, in 2000 in Uganda, saw 425 people infected, of which just over half died.
How likely is it to reach Britain?
There has been a previous case in the UK, when in 1976 someone was accidentally infected in a laboratory, but survived.
At the start of the outbreak, the cases were mainly in remote areas affecting rural workers unlikely to take flights and spread the disease internationally.
However, the disease is now spreading beyond Guinea, Liberia and Sierra Leone and Nigeria into the United States and Europe.
A nurse in Spain became was confirmed as having the virus on October 6, the first person known to have caught the virus outside Africa. Maria Teresa Romero Ramos, 44, treated one of two Spanish missionaries who contracted the disease while working in West Africa and was repatriated to Spain.
Liberian national Thomas Eric Duncan, the first person diagnosed with the deadly virus on US soil, is fighting for his life in a Dallas hospital.
Freelance NBC cameraman and US-British citizen Ashoka Mukpo is being treated at the Nebraska Medical Center after being flown back from Liberia.
Travellers who may have been exposed to the Ebola virus should seek urgent medical attention immediately if symptoms develop within 21 days of coming home. UK doctors who suspect Ebola can get expert advice from the Imported Fever Service.
Public Health England says the risk of a traveller contracting Ebola is very low without direct contact with the blood or body fluids of an infected person or animal.