A woman who died of Ebola this week in Sierra Leone potentially exposed at least 27 other people to the disease, according to an aid agency report.
It raised the risk of more cases just as the deadliest epidemic on record appeared to be ending.
Just a day earlier, the World Health Organisation (WHO) had declared that "all known chains of transmission have been stopped in West Africa".
This meant that the region was officially free of the disease after a two-year epidemic that killed more than 11,300 people.
The WHO warned, however, of potential flare-ups, as survivors can carry the virus for months.
The new case in Sierra Leone is especially disquieting because authorities failed to follow basic health protocols, according to the report seen by Reuters.
It was compiled by a humanitarian agency that asked not to be named.
The victim, a 22-year-old woman, began showing symptoms at the beginning of the year, though the exact date is unknown, the report states.
A student in Port Loko, the largest town in Sierra Leone's Northern Province, she travelled to Bamoi Luma near the border with Guinea in late December.
By the time she travelled back to her parents' home in Tonkolili district, east of the capital Freetown, using three different taxis, she had diarrhoea and was vomiting, the report said.
She was nursed by members of a household of 22 people.
She sought treatment at a local hospital on 8 January where a health worker, who did not wear protective clothing, took a blood sample. It was not immediately clear whether the sample was tested for Ebola.
She was treated as an outpatient and returned home, where she died on 12 January.
Health workers took a swab test of her body following her death, which tested positive for Ebola.
Asked about apparent errors in handling the case, a health ministry spokesman said that the patient had been tested for the virus and had received treatment in a government hospital.
He did not give further details.
Almost all the victims of the regional epidemic, which originated in the forests of Guinea in 2013, were in Sierra Leone, Guinea and Liberia.
All three nations have been declared free of the virus at various times.
But both Sierra Leone and Liberia have seen the disease return despite passing a 42-day period with no new cases, after which countries are declared free of Ebola transmission.
Ebola is passed on through blood and bodily fluids, and kills about 40% of those who contracted the virus.
While the WHO has said that another major outbreak is unlikely, it stated that there was a risk of flare-ups throughout this year because of the way the virus can persist in those who survive it.