The Scottish nurse who has contracted Ebola is to be given an experimental drug.
Dr Michael Jacobs, treating her in an isolation tent at the Royal Free Hospital in North London, said the unnamed medicine is experimental and "not proven to work".
The 39-year-old, the first person to be diagnosed with Ebola in the UK, has also received convalescent plasma from donors who survived the disease.
It contains antibodies that doctors hope will help her fight the virus, which she contracted while treating Ebola patients in West Africa.
Dr Jacobs told a news conference: "She is sitting up and talking. She is able to read.
"She's been eating a bit, drinking and she's been in communication with her family, which has been really nice.
"She's as well as we can hope for at this stage of the illness. She's had the treatment, it's gone very smoothly, no side-effects at all."
Ms Cafferkey, a public health nurse at Blantyre Health Centre in South Lanarkshire, was part of a 30-strong team treating Ebola patients in Sierra Leone.
She fell ill hours after she was given the all-clear at Heathrow Airport, despite raising fears about her temperature with officials at the airport.
Her experience has prompted questions about the effectiveness of UK Ebola screening, with procedures at Heathrow branded "chaotic" by her colleagues.
Dr Jacobs said the Royal Free Hospital was unable to obtain ZMapp, the drug used to treat fellow British volunteer nurse William Pooley, because "there is none in the world at the moment".
"Although it's been used in a handful of patients, we simply don't know if ZMapp works and is of benefit to patients," he said.
He did not name the anti-viral drug she has agreed to take, but said: "There's obviously very good reason to believe it's going to help her, otherwise we wouldn't be using it at all, but we simply don't have enough information to say that's the case.
"The one thing I'll say about the drug - it has been used extensively in people previously for different reasons, and it has a very good safety record in humans which has encouraged us to use it in this experimental way."
He added: "We are naturally cautious, particularly when we are aware that this disease is so variable in what it does.
"I make no prognosis about the next days, until we see what happens."
He said a lot had been learned about Ebola, but stressed the disease was unpredictable and there was no proven cure: "We are left with that uncertainty, and we are just going to have to take this day by day."
Dr Jacobs said Ms Cafferkey's family are unable to touch her, but can talk to her through an internal communication system.
Originally published: 15.27
