Down with the Sickness

Shinichi Izumi, best friend to Yuji Shokobu. Both have been best friends since kindergarden, and haven't seen each other since freshmen year of highschool, so they never were connected. But one day, a mysterious virus spreads, similar to the parasytes that invaded the brain, but now it's an entirely different walk in the park, Yuji finds a parasyte that hasn't been infected yet, but since the virus only spreads through human infected parasites, and hostless parasytes, the parasyte invades Yuji's left hand.
Cleverly named Hidari, Yuji makes a friend that vows to protect him and discover what's behind this virus infecting both humans and parasites alike.
(Disclaimer, I do not own the rights of the anime/manga created by Hitoshi Iwaaki, I also do not own the rights of Disturbed's song "Down with the Sickness". Anything and Everything involved with Parasyte the Maxim is purely non cannon.)