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Source: WildList Org.
The WildList (Worldwide)
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.