A patient in Liberia has tested positive for the Ebola virus, health officials said Friday, more than two weeks after the last known case in the country had been discharged from the hospital.

The news deflated optimism that Liberia, one of the three West African countries hit by the Ebola epidemic that has killed more than 10,000 people since it began a year ago, would soon be officially declared free of the virus.

The patient, a 44-year-old woman from the Caldwell area near Monrovia, the capital, first developed symptoms around March 15, said Dr. Moses Massaquoi, leader of the Clinton Health Access Initiative in Liberia and national case manager of the Ebola response.

Health officials said it was unclear how the woman, a food seller, had been infected. She had not been on a monitoring list for possible exposure and she said she had not traveled outside Liberia. The Information Ministry issued a statement saying “initial suspicion is that it may be the result of possible sexual intercourse with an Ebola survivor.”