So, in short, The Hysteria Virus is a virus that infects people who are highly mentally stressed, and nearly at their breaking point. It was accidentally created by Spark, who didn't actually mean to do so at the time.
Longer explanation under the cut. WARNING, however, BODY HORROR and TRYPHOBOIA,
It behaves a lot like how the stuff from the Upside-down that the Mind Flayer controls in Stranger Things does. The hysteria Virus spreads to anything it can to gain control and is a hive mind. It talks to Spark a lot, trying to convince him that everyone he knows is against him and wants to kill both him and the virus.
Once the virus has successfully infected someone by forming in their brain, it begins to slowly show signs of existing. It starts off with vivid red patches of skin, auditory hallucinations, and the Hysteria Virus talking to the infected to reassure them that they are safe with the virus and that no one has to know about it. It slowly starts forming eyes in laces that shouldn't be eyes on the infected's body- usually their hands, neck, ankles, and smaller eyes around their normal set of eyes. In later stages, it connects the infected to the hive mind, forming more eyes around the infected's body, more vivid red patches of skin, and forms eyes inside of the infected's body (inside of throat, mouth, and on organs)
Spark, being the Hysteria Virus's host, is the main entity that the Hysteria Virus responds to. He can't control if it will infect someone in particular, but he can influence it to spread and has the ability to create a crystalline substance with it. Spark can also see out of any eye that the Hysteria Virus has created, and can communicate with anyone else who is infected as long as that person had already gotten to the stage where they can hear the Hysteria Virus.
At this point, the Hysteria Virus has latched completely onto Spark's brain, being able to make him hallucinate whatever it wants and, if anyone tried to kill the Hysteria Virus in him, it would kill Spark too. It's far too deeply sunken into Spark's brain to remove at this point.
