The 10 scariest computer viruses of all time

Anna Kournikova (2001)

The Anna Kournikova virus is so named because it tricked its recipients into thinking they were downloading a sexy picture of the tennis star. Financial damages associated with Kournikova were limited, but the virus had a big pop culture impact: It became a plot point in a 2002 episode of the sitcom Friends

Sasser (2004)

In April 2004, Microsoft issued a patch for a vulnerability in Windows’ Local Security Authority Subsystem Service (LSASS). Shortly after, a teenager in Germany released the Sasser worm to exploit the vulnerability in unpatched machines. Multiple variants of Sasser took out airline, public transportation, and hospital networks, causing $18 billion in damages.

Melissa (1999)

Named after a Florida stripper, the Melissa virus was designed to propagate by sending itself to the first 50 contacts in its victims’ email Outlook address book. The attack was so successful that the virus infected 20% of the world’s computers, causing an estimated $80 million in damages.

Virus creator David L. Smith was caught by the FBI, served 20 months in jail, and paid a $5,000 fine.

Zeus (2009)

While many malware programs on this list are little more than nuisances, Zeus (aka Zbot) was a tool used by a complex criminal enterprise.

The trojan used phishing and keylogging to steal online banking credentials, draining a cumulative $70 million from the accounts of its victims.

Storm Trojan (2007)

Storm Trojan is a particularly sinister piece of email-distributed malware that accounted for 8% of all global infections just three days after its January 2007 launch.

The trojan created a massive botnet of between 1 and 10 million computers, and because it was designed to change its packing code every 10 minutes, Storm Trojan proved incredibly resilient.

ILOVEYOU (2000)

The ILOVEYOU worm (aka Love Letter) disguised itself in email inboxes as a text file from an admirer.

But this "love letter" was anything but sweet: In May 2000, it quickly spread to 10% of all internet-connected computers, leading the CIA to shut down its own email servers to prevent its further spread. Estimated damages were $15 billion.

Sircam (2001)

Like many early malware scripts, Sircam used social engineering to trick people into opening an email attachment.

The worm chooses a random Microsoft Office file on victims’ computers, infects it, and sends it to all the people in the victims’ email contact list. A University of Florida study pegged Sircam cleanup costs at $3 billion.

Nimda (2001)

Released just after the 9/11 attacks, many thought the devastating Nimda worm had an Al Qaeda connection, though that was never proven, and then-U.S. Attorney General John Ashcroft denied any correlation.

It spread via multiple vectors, bringing down banking networks, federal courts, and other key computer systems. Cleanup costs for Nimda exceeded $500 million in the first few days alone.

Michaelangelo (1992)

The Michelangelo virus spread to relatively few computers and caused little real damage. But the concept of a computer virus set to "detonate" on March 6, 1992 caused a media-fueled mass hysteria, with many afraid to operate their PCs even on anniversaries of the date.