Actual viruses spreading in the wild.
Summer of 1999, the start of many more mass-mailers.
Mass-mailers immediately make it into the
wild, being seen all over the place right away.
ITW status has a 1-yr shelf life, with a 1-3 month bookkeeping (reporting)
delay.
If it's not seen for a year, it
goes off ITW.
Thus, the decline
recently.
But slope of decline is less
than slope rise in 2000, meaning more new viruses are replacing the ones that
die.
Mass-mailers die off quicker than slow mailers.
So, effectively, a lot of mass-mailers will
drop off the ITW list 15-18 months after discovery.
So, as the chart is starting to show, the
decline is ending.