For the first time, scientists have found evidence that a woman can harbour the Ebola virus for more than a year and then infect others.

The discovery involved transmission within a Liberian family in the closing days of the West African epidemic that lasted from December 2014 to mid-2016. More than 28,600 people were infected and 11,325 died.

The episode raises new medical questions: Scientists do not know how the virus hid inside the woman for 13 months before re-emerging in lethal form.

However, because she fell ill soon after giving birth, experts believe the immune suppression that normally occurs in pregnancy may have triggered a relapse.

The case, published Monday by Lancet Infectious Diseases, also worries experts because fear of Ebola survivors and discrimination against them persists in Africa, and the notion that a female survivor can pass on the virus could worsen that.

In the past, although bits of virus have been found in the breast milk and spinal fluid of women who recover, only male survivors have been shown to infect anyone. Those transmissions were through sex — the virus is known to persist in semen for two years.

The family cluster came to light in November 2015, six months after Liberia had declared its outbreak over — prematurely, it turned out.

A 15-year-old boy, the woman’s oldest son, was hospitalized vomiting blood. He tested positive for Ebola and, despite intensive treatment, died 10 days after his symptoms first appeared.

As soon as he tested positive, contact-tracers brought his whole family, including his 33-year-old mother, his 40-year-old father and his three younger brothers, aged 8, 5 and two months, in for observation and testing.

The family also became the first participants in a clinical trial of a new Ebola vaccine, said Dr. Emily Kainne Dokubo, an epidemiologist at the U.S. Centers for Disease Control and Prevention who was then the leader of the agency’s Ebola response in Liberia and is the lead author of the Lancet study.

All 120 people with any recent contact with the family were vaccinated, she said. None fell ill, and that helped prove the Merck vaccine works.

This year, the vaccine was used to defeat the most recent Ebola outbreak, which took place in the Democratic Republic of Congo. About 3,200 people were vaccinated, and new cases faded out after only about three months. On Tuesday, the director-general of the World Health Organization will officially declare it over.

In Liberia, after the 15-year-old died, blood tests showed that his father and 8-year-old brother had Ebola. With treatment, both recovered.

The 5-year-old apparently was never infected.

Neither the mother nor the new baby had virus in their blood but, rather mysteriously, both had antibodies against it. That suggested the mother had had an earlier infection and that the baby had absorbed protective antibodies through breast-feeding.

Dr. Dokubo described what she and her C.D.C. colleagues believe happened:

In July, 2014, soon after Ebola first reached Liberia from Guinea, the mother had cared for her brother, a nurse’s aide dying of an unknown illness. She was pregnant; she soon fell ill and miscarried, but slowly recovered.

Neither she nor her brother went to one of the new Ebola treatment units then being set up, so they never had Ebola tests.

“That was not uncommon, with all the stigma and fear at the time,” Dr. Dokubo said.

Thirteen months later, in September, 2015, she gave birth to a healthy boy.

Shortly afterward, in October, she was hospitalized with fatigue, shortness of breath and swollen legs. Liberia had been declared Ebola-free in May, so she was not tested for it. Instead, she was treated for malaria and given blood transfusions because she was anemic, and sent home after three days. (The hospital tested its stored blood and none had Ebola virus or antibodies, the study said.)

But pregnancy lowers mothers’ immune systems to protect fetuses from rejection, and Dr. Dokubo said pregnancy had probably unleashed a hidden reservoir of dormant virus within the woman; tests did not establish where that reservoir might be.

There was no evidence of sexual transmission; the woman’s husband and two elder sons probably became infected while caring for her, Dr. Dokubo said.

Although the case is rare and highly unusual among the thousands of Ebola survivors in West Africa, it means that countries cannot become complacent even when outbreaks seem to be over, Dr. Dokubo said.

Also, she added, survivors must be tested for Ebola if they fall ill, even if they lack common Ebola symptoms.

In an editorial accompanying the Lancet article, Lorenzo Subissi, an Ebola expert at Sciensano, Belgium’s public health institute, said the new study “could lead to additional stigma around survivors.”

Survivors are often driven out of their villages or neighborhoods because people fear they may be infectious, so Dr. Subissi suggested that vaccination be used both to stop outbreaks and to calm the fears that lead to stigmatization and abuse of survivors.