For those who haven't yet heard...
A cyber dragnet aiming to flush out the author of the MyDoom computer worm intensified on January 30, 2004 as the outbreak crippled still more e-mail networks. Investigators and security experts hoped their hunt would get a boost after Microsoft Corp. offered a $250,000 reward for information leading to the arrest and conviction of the creator of one variant, MyDoom.B
To summerise. Thick idiots opening attachments have caused this problem to the point where it is now the worst email worm incident in history. These people are probably the same idiots who 'don't believe in antivirus software' as they're only on 'now and again'.
Can everyone please ensure you only open attachments you know to be safe, or has been confirmed by the sender. Update your virus settings on a REGULAR basis or you'll be one of the lucky few who learn the hard way.