Scottish health authorities were last night conducting checks on her friends and family as a precaution.
Miss Cafferkey, who appeared on the ITV programme Lorraine ten days ago, is believed to be suffering a severe form of what scientists have called "post-ebola syndrome".
Doctors are beginning to realise that the virus can linger in the body, causing symptoms months after patients are given the all-clear.
Miss Cafferkey, who had returned to duties as a community health nurse in South Lanarkshire, was invited to Downing Street after winning an award for her humanitarian work. She had also made a school visit on Monday to give a talk and thank pupils who raised money for ebola charities.
Letters of reassurance were sent to parents of pupils at Mossneuk Primary School, East Kilbride, while Downing Street is believed to have been told that Mrs Cameron will not require testing.
But parents at Mossneuk Primary remain concerned. David Cherry, 36, an engineering manager whose seven-year-old twin boys were at the talk, said: 'I think she obviously has got a great story to tell but how do I feel about her coming into a primary school? I think we should have been informed.'
Yesterday a spokesman for the Royal Free said that Miss Cafferkey was in a "serious condition", adding: "She will be treated in isolation in the hospital's high-level isolation unit under nationally agreed guidelines."
Professor Paul Cosford, medical director of Public Health England, added: "It is important to remember that the ebola virus can only be transmitted by direct contact with the blood or bodily fluids of an infected person while they are symptomatic. The risk to the general public remains low."
Earlier this month Miss Cafferkey revealed she had felt like "giving up" as her condition became critical last year. And last month she said was suffering sore joints and hair loss, common in half of patients.
- Daily Mail