The Navidad virus appeared for the first time two years ago, around Christmas time. Now it's back in a new shape and no anti-virus program is able to stop it yet, according to virus analysts from MessageLabs. Most attacks were reported in the UK.
"This file has been compressed into a different format - essentially just packaged differently - so a lot of the scanners just didn't pick it up. It's been fooling a lot of people", said Mark Tosh, virus expert from MessageLabs.
The virus uses e-mail addresses from Outlook address book to spread messages, causing overload and crash of mail servers. "All the major anti-virus vendors are now detecting it, so users need to ensure their anti-virus software is up-to-date."