Onel A. de Guzman, a Filipino computer student, right, and his sister Irene listen to a reporter's question during a news conference at their lawyers office in Manila on Thursday May 11, 2000. // The ‘love’ bug (Ed Wray/AP Photo)

Larger image

Onel A. de Guzman, a Filipino computer student, right, and his sister Irene listen to a reporter's question during a news conference at their lawyers office in Manila on Thursday May 11, 2000.

When: 2000

Even as recently as 2000, our naïveté was apparent online. That year, a Filipino computer student released the "love" worm, a virus that was emailed to unwitting users under the heading: "ILOVEYOU." Attached to the email was a virus file titled "love letter," which contained a malicious program Internet surfers unsuspecting downloaded when they opened the attachment. The "love" bug crippled the email systems of the Pentagon, CIA and British Parliament, and cost companies and other government agencies a total of $15 billion to remove.

Lesson learned: Don't download email attachments from unknown senders.

* Bing: The 12 costliest computer viruses

* Video: What caused TV reporter's on-air confusion?