Carcossa is one of the most nefarious, having no immediate effects, capable of spreading undetected through a population before spontaneously turning the community into a cult.
Each victim of the strain (usually called dreamers) halts their usual dream activity and begins to dream only one dream, the same every night. Dreamers are typically dissatisfied with their attempts to describe their dreams to others, feeling that key details, moods, and inferred contexts are lost. Of course, people have the problems describing their normal dreams to others, so perhaps this is normal, and the dreams are not so alien after all.
When dreamers meet, they inevitably wish to discuss their dreams, and will sometimes find that their dreams are linked, and that they dream to perspectives of the same scene. Dreamers who have successfully linked dreams begin to hold the conviction that their dreams are fragments of the true world, and that this world is the dream. Compilations of dreams suggest a narrative of an immortal society, fallen past decadence but unable to die. Each dreamer dreams one of the days of the lives of its residents, enacting pointless politics and ritualized betrayals.
The strain is memetic, but how it is spread is an unsolved problem, a fact that causes Firewall no end of worry.
The dream is a memetic strain, spread by basilisk hacks, infecting all humans.
Stage 1: (Initial infection to one month) During this stage the strain is dormant. It can only be detected and treated through psychosurgy.
No effects during this stage.
Stage 2: (One month+) At this stage the dreamer begins to dream their dream. They dream the same dream every night, and will dream no other dreams. The victim is not otherwise effected, and carry out their daily lives as normal.
Victims take 1 SV per day during this stage.
Stage 3: (Upon linking dreams) It is if the dreamer is introduced to other dreamers that the strain enters its most active phase. Dreamers will often find that their dreams are linked; that another dreamer is dreaming the perspective of one of the characters in their own dreams. Linked dreamers will seek others and often form the nucleus of a cult of linked dreamers. Dreamer cults seed their locations with camouflaged basilisk hacks, the only way the strain is spread.
Victims take 1d10/2 SV per day, gaining disorders that parallel the personalities of the character they dream.