Which virus caused the first major email worm outbreak in 2000?

The ILOVEYOU worm caused the first major email worm outbreak in 2000.

Also called the Love Bug or LoveLetter, ILOVEYOU began spreading on 4 May 2000 from Manila, Philippines. It arrived with an enticing subject line and an attachment named LOVE-LETTER-FOR-YOU.TXT.vbs. On systems configured to hide known file extensions, the attachment could appear to be an innocent text file.

Written in VBScript, the worm used Microsoft Outlook address books to mail copies of itself to a victim’s contacts. That made messages appear to come from acquaintances, an early and powerful example of social engineering. It could also overwrite files and attempt to download a password-stealing Trojan.

ILOVEYOU infected tens of millions of Windows computers and disrupted corporate and government email systems worldwide. It is often called a virus, but technically it was a worm because it replicated and spread automatically across networks. Its outbreak helped expose the security risks of trusting email attachments.