E-mails containing a virus and falsely identifying the sender as the National Institute of Infectious Diseases have been sent out amid the swine flu epidemic, the institute said Thursday.

Attached to the e-mails is a file in Japanese named "knowledge on swine influenza.zip," containing a virus that can damage Windows-based computer systems when opened.

"We got information from an outside person about the e-mails," said one of the institute's officials, who asked not to be named. "But we don't know how many people have been affected by viruses because of it."

The e-mail, which has a Yahoo domain, warns that swine flu continues to spread worldwide and that some people in Japan may already have been infected.

"To protect our own lives, people should be aware of basic information on swine flu," says the e-mail, which at the bottom identifies the institute as the sender.

The institute regularly updates swine flu data on its Web site and said none of its e-mails has a Yahoo domain.

The institute uses the nih.go.jp domain.

In addition, the institute puts all of its information on its Web site and does not provide e-mail services, such as sending out information on diseases, it said.

The institute asked all users who have received the e-mail to delete it without opening it.