Did you know that Parasyte inspired Tokyo Ghoul? I love Parasyte but can't say I'm all that fond of TG, so here's my take on what happened to bring about Ghouls if they existed in the same timeline as Parasytes.
In the decade after the initial Parasyte epidemic the foreign organisms ingrained themselves into humanity. Descendants of infected people developed stronger bodies, greater tolerances for poison and pain, as well as a taste for flesh. The longer the Parasyte's DNA stewed in the human bloodstream, the greater and more pronounced the mutations became; and the hunger grew with them. Governments around the world had tried to control and weaponise the first parasytes, but were slow and unprepared to deal with the side effects of those who slipped through their fingers. Roughly 180 years later the public started catching on that there were inhuman people in their midst. As murders and cannibalism started to rise in cities and towns across the world the name Ghoul found itself attached to the perpetrators. The US was somehow able to clamp down on the situation through the publication of information, which overwhelmed the Ghouls with sheer numbers. Nearly 200 years after the day the world changed, the world is still changing.