Dozens of people linked to the most recent cases of Ebola in Sierra Leone are still missing, the World Health Organization warns, saying that among them 18 contacts are at high risk of having the virus. WHO representative Dr. Anders Nordstrom said that while some 70 people under quarantine in three northern Districts, Kambia, Port Loko and Tonkolili, are being released, more than 48 contacts are missing.

“We need to strengthen surveillance efforts to identify and investigate all patients with symptoms meeting the case definition in hospitals and communities and to investigate all deaths in the Kambia District and all districts,” he said Wednesday. The world health agency will continue to ensure that all deaths reported are swabbed and tested until June 2016 to prevent a recurrence of the virus, he said.

A young woman tested positive for Ebola after she died. She was given a traditional burial because Ebola wasn’t suspected, which is significant because in such burials people touch the dead body and that can pass Ebola. Her aunt later tested positive for Ebola. The 38-year-old aunt is responding to medical treatment, he said, while four others will remain under close monitoring in quarantined homes.

Ebola is spread through direct contact with the bodily fluids of victims, and corpses are especially contagious. Traditional funerals in the region where mourners touch the body were a major source of virus transmission during the epidemic in Sierra Leone, Liberia and Guinea.

Nearly 4,000 people died before Sierra Leone was first declared free from transmission on Nov. 7. The West African outbreak of Ebola has killed more than 11,300 people, mostly in Sierra Leone, Liberia and Guinea, since late 2013.