The Kama Sutra worm, which targets Microsoft, Adobe Systems and ZIP files, is a threat because many users will not know that the virus has infected their computers until it is too late, security specialists said on Tuesday.

They also estimate that the worm - which spreads by emailing itself to addresses in an infected computer's mailbox - may already have slipped on to 275,000 to 500,000 machines and is now waiting to obliterate files on Friday.

The virus, also known as Grew.A or MyWife, tricks users by appearing as an email attachment with subject lines such as Hot Movie, give me a kiss and Miss Lebanon 2006.

Some variations refer to the ancient Kama Sutra guide to elaborate sexual positions in order to attract attention and convince victims to open it.

Racy content

"It claims to be a movie or picture with some sort of sexual content," said Johannes Ullrich, chief research officer at the nonprofit SANS Institute research group. "That is how it tricks you."

The virus causes a keyboard and mouse to freeze up and then disables anti-virus programs when the computer is restarted, leaving a machine vulnerable, said Ken Dunham, rapid response director at VeriSign Corp's  security unit iDefense. The attack is scheduled to begin at midnight on 3 February.

"If you have opened an email and your computer froze up, you should be very concerned"

Ken Dunham,
computer expert

The virus has mainly infected computers of vulnerable consumers and small businesses, which are far less likely to have up-to-date security software, he said.

The Kama Sutra worm also stands out because its primary purpose is to destroy files rather than to seek financial gain or to take control of a computer, security experts said.

Dunham said any users who suspected that they might have triggered the worm should reinstall an anti-virus program and make sure the virus has been removed.

"It is already under way and will be activated unless people get removal tools," he said. "If you have opened an email and your computer froze up, you should be very concerned."