Psychosis is a Tier 5 symptom found in the Neurax Worm DLC and the Necroa Virus DLC. It is also a Tier 1 symptom in the Mad Cow Disease scenario.
The variation of this symptom allows the Neurax Worm to tweak the dopamine receptors in the hosts' brains, inducing psychosis and personality shifts to better accommodate the worm, such as hating headwear and long hair.
The variation of this symptom, causes psychotic episodes of extreme violence, which can be lethal. It also slows cure research.