A Japanese woman who recently returned from Congo underwent testing at a Tokyo medical institution for a possible Ebola virus infection, but tested negative on Sunday, the health ministry said.

The Saitama Prefecture woman in her 70s was hospitalized at the National Institute of Infectious Diseases after developing a high fever on Saturday, the Ministry of Health, Labor and Welfare said.

Her fever went up to 39.2 C on Saturday night, it said. The virus can be deadly if it goes untreated, but no one in Japan has so far been confirmed to have contracted the disease.

The ministry said the woman, who returned to Japan on Wednesday, has tested positive for influenza, adding she did not have any contact with Ebola-infected people during her stay in Congo, formerly known as Zaire, for work.

After receiving information on the possible infection, the government set up a liaison unit at the prime minister's office.

At the same time, the office, also providing information in English on Twitter, issued a reminder that Ebola hemorrhagic fever is contagious only when its symptoms become evident, so that the risk of contracting the disease simply by having been on the same plane with the patient is "extremely low."

The disease is also not an airborne infection, according to health authorities.