The human body is not a mere brain-driven machine of nerve and bone. Every subsection of its anatomy is an independent ecosystem of organisms (or organism-like structures), each with their own motivations and metabolisms. Though much of this zoology occurs beneath a veil of skin, it is still a phenomenon that can be seen with the naked eye. When one looks upward into cloudless daylight, they just might see the spectral outlines of lifeforms known as lucigens.
These symbiotes are directly invested in the behavior of their hosts. Though not consciously visible under normal conditions, they are faintly bioluminescent, and glow softly to form echoes of things previously seen, favoring forms that have been known to trigger an increase in pulse. In this way, through subliminal flickerings that suggest the fulfillment of lust and other vices, they draw the rest of the body towards the sunlight upon which they feed. The afterimages in use can be seen by the host briefly upon consciously blinking, though they typically dissipate too quickly to discern.
Too much solar radiation is lethal for these colonies, and gazing directly into any sufficiently bright light source results in their deaths en masse. The iridescent clouds witnessed as their colonies sink to the bottom of the eye are commonly referred to as "phosphenes." Children are often more fascinated by this phenomenon than adults, and commonly stare at light bulbs out of curiosity, or even press their fingers into their own closed eyes, forcing their body’s lucigens to panic and produce sudden, elaborate patterns.
Due to the relative isolation of their populations beneath the eye’s sclera, numerous mutant lucigen strains have emerged over the course of the last century, naturally selected into ubiquity by the habits of their hosts. Many common variants prefer the radiance of liquid crystal, or the blaze of cathode rays, fueled by the harsh glow of the digital age.
Other, more rare lucigens, have highly specific cravings that can only be fulfilled in the complete absence of sunlight. These colonies hunger exclusively for things like a sodium streetlamp’s orange, or a full moon’s sliver, or even for one particular bend of neon. Those who carry them pass one another on the street at night, drawn together along lepidescent paths, unaware of the forces compelling them. Every so often, two such wanderers meet each other's gaze, and feel that they are connected in a manner that they cannot explain; then, after a moment’s hesitation, something else inside them both compels them further into the dark.