We keep telling children, "Don't talk to strangers!" Likewise, we need to warn adults, "Don't click on stuff from strangers!" Over the last couple of weeks, more than 100 Windows computers on campus were infected with a dangerous computer worm because people were taken in by e-mails claiming to offer a Hallmark e-card from a friend or promotions from McDonald's or Coca-Cola.

When the victim opened an attachment to the message, it installed a program that logged keystrokes and could send usernames, passwords and other sensitive information to the originators of the malware. It includes a "backdoor" that will let a remote user take control of the computer.

The malicious program is dubbed a "worm" rather than a virus because it spreads itself. It scans the infected computer for e-mail addresses and sends copies of itself to all those addresses. It also can spread through devices attached to the computer's USB ports such as flash drives and cameras.

Some malware gives itself away with poor spelling and grammar, as in "Get all Coca Cola drinks for free in the rest of your life." But a real tipoff is that the message comes with an attachment. "You would never see an e-card as an attachment," said Aaron Wade, senior security engineer in Cornell's IT Security Office. "Looking at links you should always be cautious, but [an attachment] is a sure sign that something is wrong."