DAKAR, Senegal (AP) — A World Health Organization spokeswoman says that Ebola experts are in Sierra Leone's Kambia district investigating a case that emerged less than a week after the country's last known patient was discharged from hospital.

Dr. Margaret Harris said Monday that once the source of transmission is found and contacts are traced, a vaccination trial will also begin in northern Sierra Leone.

Authorities on Sunday said samples from the body of a 67-year-old woman who died in Kambia recently came back positive for Ebola.

Harris said they are investigating how the woman got the deadly virus, concerned with the possibility of a silent chain of transmission.

The worst Ebola outbreak in history has killed nearly 4,000 people in Sierra Leone and more than 11,300 overall.