RTFA. If I didn't explain why I'd be flaming you so here goes:
Anti-Virus "experts" always assumed that they could apply biological models to computer viruses. But Mellissa != Polio. The reason (which they just figured out) is because there's always a computer that isn't protected but is connected to an infected computer, via the internet. I guess it's statistically inevitable.
The signifcance of it is that those small percentage (less then 1%) Mellissa infections maintain a persistence unlike meatspace viruses, which eventually go away when the numbers are that low. You have to admit that changes the scene a little for AV makers.
That's really the basic rundown of it but NewsBytes.com has a better write up of it . It seems more geared towards people like us. SecurityPortal reported this about a week ago. But check out the link I posted. Maybe it wont seem so boring. "Me Ted"
The world is shocked and overwhelmed by this revelation, and people finally can stop worrying about getting a nasty case of "Melissa".
Please, this isn't news.