Britain's first confirmed Ebola sufferer, William Pooley, has been treated with experimental drug ZMapp.
Mr Pooley is being treated at the Royal Free Hospital, London, and Dr Michael Jacobs, consultant and clinical lead in infectious diseases at the hospital, said: "We have had the opportunity to give him the ZMapp treatment.
"It is an experimental medicine, we made that absolutely clear in our discussions with him."
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The volunteer nurse from Eyke, Suffolk, was exposed to Ebola while working with patients in Sierra Leone.
ZMapp has been dubbed by some as the "cure" after two aid workers from America were successfully treated for Ebola after taking it.
Dr Jacobs added: "What has become apparent to us is he is clearly a rather resilient and remarkable young man."
The hospital said Mr Pooley was "sitting up and talking to the nurses and doctors who are looking after him".
He was given the first dose of ZMapp on Monday and further doses are expected to be given "in due course".
Dr Jacobs added: "We are giving him the very best care possible. However, the next few days will be crucial. The disease has a variable course and we will know much more in a week.
"What we want to do is give him the best chance we can and that's why we used the ZMapp.
"He considered his options very, very carefully. It is an experimental drug. He wanted to weigh up what we knew about it and he came to the very clear conclusion in his own mind he would like to go ahead with the treatment."
He said it was "too early" to say what impact the drug has had but added: "Pleasingly, it seems to have had no side effects at all."
Health Secretary Jeremy Hunt said Mr Pooley was being treated by "world class" doctors and added: "I'm proud of what the NHS has been able to offer to a very, very brave man."
There is no cure for Ebola but with treatment of the symptoms, and proper hydration, patients have a chance of survival.
The virus is spread between humans through direct contact with infected bodily fluids.