LONDON — A British nurse diagnosed with Ebola is in critical condition at a London hospital after receiving blood plasma treatment and taking an experimental drug.

The condition of Pauline Cafferkey, a Scottish nurse who returned from helping to fight the virus in Sierra Leone, gradually deteriorated over the past two days, the Royal Free Hospital said in statement on its website Saturday. She had been sitting up in bed, talking and reading on Dec. 31 and had decided to take an anti-viral drug.

Cafferkey is a National Health Service nurse who was helping to fight the Ebola outbreak at a treatment center run by Save the Children in Kerry Town, Sierra Leone. More than 20,000 people have been infected in the outbreak, mostly in three African countries, according to the World Health Organization. Almost 8,000 have died.