Ebola nurse William Pooley returns to Sierra Leone

William Pooley, the British nurse who contracted Ebola while volunteering in West Africa, has returned to Sierra Leone to resume his work.

He said there was a "real emergency" in the country and he was "delighted" to be back on the front line.

Mr Pooley will start work at a hospital in the Sierra Leonean capital, Freetown, on Monday.

Having recovered from Ebola, his immune system should be able to fight it but the extent of any immunity is unclear.

Doctors are not certain how long his immunity will last or whether he is completely resistant.

But Mr Pooley said: "It's massively safer for me to work there now than it was before."

Can you catch Ebola twice?

It is unclear if someone can contract Ebola twice.

If a person has successfully fought off the infection, it means their body has learned how to combat the virus.

Scientists do not know how long this immunity lasts - it could be lifelong or it may wane.

English nurse William Pooley, who recovered from Ebola after receiving the experimental drug ZMapp, has donated some of his blood in case it offers clues about how to kill the virus.

The Spanish nurse who became the first person to contract Ebola outside West Africa tested negative for the virus after reportedly receiving human serum containing antibodies from Ebola survivors.

A recent study in the New England Journal of Medicine of survivors of the Ebola virus outbreak that occurred in 2000 in the Gulu district of Uganda suggests immunity lasts at least 12 years.

'Unnecessary deaths'

While in the UK, the 29-year-old had said he was "impatient" to return to his work in Africa and would travel as soon as he had a new passport. His old one was incinerated when he was evacuated.

The nurse, from Eyke in Suffolk, was flown back to the UK by the RAF on 24 August and was taken to the UK's special isolation unit at the Royal Free Hospital in London.

Mr Pooley has been treated at a specialist isolation unit at London's Royal Free Hospital

He was treated during the early stages of the infection, before the onset of internal or external bleeding.

Unlike many people in Africa he was given the experimental drug ZMapp, which seemed to lower levels of the virus in his body.

He has since made a full recovery and has donated his blood for medical research.

Mr Pooley said: "I would like to once again thank the team at the Royal Free Hospital and the RAF who provided me with such excellent treatment and support.

"But the real emergency is in West Africa, and the teams out there need all the support we can give them - I am now looking forward to getting back out there and doing all I can to prevent as many unnecessary deaths as possible."

More than 4,500 people have died in the Ebola outbreak, which is the largest in history.

Mr Pooley will work at the isolation unit at Connaught Hospital in Freetown, Sierra Leone, where he will train staff and set up new isolation units.

He will work with a team from King's Health Partners - a collaboration between King's College London and three NHS trusts - which is operating in the country.

Dr Oliver Johnson, programme director for the King's Sierra Leone Partnership, said: "It is fantastic that Will has chosen to join our small team here at Connaught Hospital.

"The situation here in Freetown is getting worse by the day and so Will's experience and commitment will be vital as we do everything we can to stem the flow of cases.

"The best way of stopping Ebola spreading even further is to fight it at its source and I look forward to working with Will to do just that."

How not to catch Ebola:

• Avoid direct contact with sick patients as the virus is spread through contaminated body fluids

• Wear goggles to protect eyes

• Clothing and clinical waste should be incinerated and any medical equipment that needs to be kept should be decontaminated

• People who recover from Ebola should abstain from sex or use condoms for three months

Ebola basics: What you need to know

How Ebola attacks

What virus has hit - in maps

Uncertainty over figures