“It’s reminded people that the consequences go on so much longer for survivors,” said Dr. Bruce Aylward, who heads the World Health Organization’s Ebola effort. “It’s really spurred concern we make sure the needs of these people aren’t lost.”

The Royal Free Hospital, which last Friday referred to Ms. Cafferkey’s illness in a news release as “an unusual late complication of her previous infection,” said Wednesday that she was being treated for Ebola. The virus, several experts said, managed to somehow persist and apparently re-emerged to cause a severe disorder of her central nervous system. Dr. Aylward said her spinal fluid had tested positive for traces of Ebola.

“This isn’t a recurrence of Ebola hemorrhagic fever; this is clearly a meningitis-like syndrome, a neurological syndrome, which is a result of the lingering of Ebola virus,” said Stuart T. Nichol, chief of the viral special pathogens branch at the Centers for Disease Control and Prevention in Atlanta. He stressed that the complication appeared to be extremely rare.

“We’d think the likelihood of these types of events is probably higher with cases where patients are very severely, critically ill,” Dr. Nichol said. He theorized that very high concentrations of virus in the blood could seed areas of the body that are harder for the immune system to reach.

Ms. Cafferkey had seemed healthy until very recently and went to a clinic at a hospital after complaining of feeling sick, according to British media reports. The clinic sent her home, the reports said.

Clinicians following patients in West Africa have documented a range of more common complications in survivors. “Body aches, joint pains, eye problems, and ear problems,” said Audrey Rangel, field coordinator for the International Medical Corps in Kambia, Sierra Leone, which helps run a clinic for Ebola survivors.

Col. Foday Sahr, commanding officer of the joint medical unit in Sierra Leone, said that of 290 registered Ebola survivors being followed weekly at a military hospital in Freetown, one or two exhibited neurological symptoms including weakness on one side of the body, but that no delayed cases of encephalitis or meningitis had been documented.