The initial suspicion is that the girl got Ebola after having sex with a survivor of the illness, but this has not yet been confirmed.
Africa's fight against Ebola is beginning to look like an unending one.
According to reports from BBC's Umaru Fofana in Freetown, Sierra Leone, a 16-year-old girl has been diagnosed with the deadly Ebola virus in northern Sierra Leone's Bomabli district, which had gone for 169 days without a case.
According to initial suspicion is that the girl got Ebola after having sex with a survivor of the illness, but this has not yet been confirmed.
The survivor was discharged after being treated for Ebola in March, according to DR Emmnauel Conteg, who is spearheading medical care for patients in Bombali.
He said 690 people in the entire village of Robuya, just outside Makeni city, had been quarantined with seven "high risk contacts" taken to a treatment unit.