The Ebola outbreak in West Africa is the world's deadliest to date and the World Health Organization has declared an international health emergency as more than 3,850 people have died of the virus in Guinea, Liberia, Sierra Leone and Nigeria this year.

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 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.

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

It was first discovered in the Democratic Republic of Congo in 1976 since when it has mostly affected countries further east, such as Uganda and Sudan. From Nzerekore, a remote area of south-eastern Guinea, the virus spread to the capital, Conakry, and neighbouring Liberia and Sierra Leone.

There have been 20 cases of Ebola being imported by someone travelling from a country of widespread transmission to Nigeria, with eight confirmed deaths. The US and Senegal have both confirmed one case each. The US Centers for Disease Control and Prevention (CDC) said in September that the virus might have been successfully contained in Nigeria and Senegal.

In October, 2014 a nurse in Spain became the first person to contract the deadly virus outside of West Africa, after treating two Spanish missionaries who had eventually died of Ebola in Madrid.

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 in neighbouring Sierra Leone.

Can cultural practices spread Ebola? Ebola is spread through close physical contact with infected people. This is a problem for many in the West African countries currently affected by the outbreak, as practices around religion and death involve close physical contact.

Hugging is a normal part of religious worship in Liberia and Sierra Leone, and across the region the ritual preparation of bodies for burial involves washing, touching and kissing. Those with the highest status in society are often charged with washing and preparing the body. For a woman this can include braiding the hair, and for a man shaving the head.

If a person has died from Ebola, their body will have a very high viral load. Bleeding is a usual symptom of the disease prior to death. Those who handle the body and come into contact with the blood or other body fluids are at greatest risk of catching the disease.

MSF has been trying to make people aware of how their treatment of dead relatives might pose a risk to themselves. It is a very difficult message to get across.

All previous outbreaks were much smaller and occurred in places where Ebola was already known - in Uganda and the DR Congo for example. In those places the education message about avoiding contact has had years to enter the collective consciousness. In West Africa, there simply has not been the time for the necessary cultural shift.

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 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, 2014, Liberia's health minister advised people to stop having sex, in addition to existing advice not to shake hands or kiss. The WHO says men can still transmit the virus through their semen for up to seven weeks after recovering from Ebola.

Fighting the fear and stigmatisation surrounding Ebola is one of the greatest challenges health workers face. But health workers themselves are becoming scared of treating patients, and are demanding better protective clothing when exposed to patients. Ebola has already claimed the lives of dozens of doctors and nurses in the

Ebola-hit region, including Sierra Leone's only virologist and Ebola expert, Sheik Umar Khan. This has put a further strain on the health services of these West African states, which have long faced a shortage of doctors and hospitals.

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. The WHO ruled in August that untested drugs can be used to treat patients in light of the scale of the current outbreak.

The experimental drug ZMapp has been used to treat several people who contracted Ebola: Two US aid workers and a Briton have recovered after taking it but a Liberian doctor and a Spanish priest have died. But the US pharmaceutical company that makes it says it has for now run out of it.

Patients with Ebola frequently become dehydrated so 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.

The current outbreak is killing between 50% and 60% of people infected. It is not known which factors allow some people to recover while most succumb.