A woman who tested positive for Ebola in Liberia last week is dating a survivor of the disease, a health official said Tuesday, offering a possible explanation for how she became the country's first confirmed case in weeks.

Ebola is typically transmitted through contact with the blood, vomit or feces of people who are sick. Survivors of Ebola are not generally considered contagious, but the virus lingers in semen, and so scientists urge survivors not to have sex for three months after recovering. Even though sexual transmission of Ebola has not yet been documented, it may be possible.

Samples have been taken from the boyfriend for testing, said Dr. Francis Kateh, acting head of Liberia's Ebola Case Management Team. The woman tested positive for Ebola on Friday, and her case has caused concern because she didn't seem to be linked to any of the people on an Ebola contacts list and said she did not travel to an infected country.

The case deflated hopes that Liberia's outbreak might soon be declared over. Liberia has seen more than 4,300 of the more than 10,000 deaths in the West African Ebola outbreak. But since it discharged its last case on March 5, it was counting down the 42 days that a country must wait before being declared Ebola-free.

"It means that we must keep up the prevention more aggressively," President Ellen Johnson Sirleaf told state radio. "It's one person that is being properly traced . so that the disease does not spread."