Well the most famous New World disease that ravaged the Old World was the Sexually Transmitted Disease Syphilis. There are very gruesome accounts you could probably look up of Europeans suffering from body horror level effects of that disease in the years after it started spreading through the Old World.
No doubt it's somewhat ironic considering the recent tangent on hentai and tentacles. (Kahhori might actually come out the victor in that scenario as well!)
But I don't think any New World disease ravaged the Old World population as much as vice versa, likely thanks to the huge urban centers stretching across Eurasia and the amount of illnesses that were transferred from livestock and other animals to humans which just didn't exist in the New World.