There are few horrors starker than meeting a wormspawn, perhaps meeting one wearing your face is the worst

Where came the sickness, the shaman knew not. No herb, no smoke, no spirit dance could carry it away. What curse of heaven carried the plague, the sages and priests could not say. The gods struck, and no prayer, no elixir could save their victims. How the malaise spread, no doctor could know. With not even a body to spread the miasma, surely it must be a curse. And when at last, the right lenses were turned upon the pus of the suffering.  

A highly underrated and underused extremely low level monster, adapted and updated for the Saerith setting

A primitive sibling of EDS, Electromagnetic Sensitivity is a rare affliction that grew to prevalence in the late Petroleum Era, and persists into the Cosmic Era. What was formerly a psychosomatic form of hypochondria is a recognized, and paradoxically, officially discredited disease.

Also known as EDS, a pervasive disorder where patients suffer from emotional instability, marked by self destructive behavior, violent outbursts, and other emotional disturbances.

Also a completely made up disorder.

The first documented virtual illness.

A mutation of Legionnaire's Disease, Space Sickness, or Space Lung, is a not uncommon malady of the Cosmic Era