The orange plague is not a sickness. It is a forgotten god reaching into the minds of bugs through their dreams.
The orange Infection that fills Hallownest looks like a disease, but it is closer to mind control. Per the wiki, it is the work of the 
Radiance reaching bugs through their dreams.
It starts quietly, planting false hopes and desires, then takes over the mind entirely. According to the wiki, the victim's will is enslaved and they become part of the Radiance's union, driven mad and aggressive.
That is why the plague has no normal cure. You cannot heal a god out of someone's head, which is the whole reason the Pale King built the Vessels to seal her instead.
This guide covers what the Infection is, the forgotten god behind it, how it visibly spreads in your own playthrough, and the only two ways the story actually stops it.
The Radiance reaching minds through dreams, not a disease.
Bugs turn aggressive and lose their sense of self.
The Crossroads worsen after a Dreamer dies or you get the Wings.
Seal or destroy the Radiance, there is no in-between.The Infection is a mental takeover, not a physical illness. Per the wiki, it "initially manifests itself through dreams, using desires and planting false hopes in the minds of bugs."
From there it grows dominant. According to the wiki the victim's mind is broken and their will enslaved, letting the Radiance control them and make them part of her union. They revert to primal instinct and turn aggressive.
The orange light and pustules you see are the surface of that control. The deeper a bug falls, the less of itself remains, until there is only the Radiance's will left moving the body.
The Dream Nail is the lens for all of this. In actual play, dream-reading infected bugs and statues is how the game shows you the plague is a dream, not a fever.
The Infection exists because the Radiance was forgotten. When the 
Pale King founded Hallownest and the bugs gave him their worship, she faded from memory but did not die.
Per the wiki, "the Radiance was not entirely forgotten, as memories of her still lingered," and bugs began to dream of her. Those dreams were the first appearance of the Infection, as bugs struggled to suppress her influence.
The wiki notes nothing worked against it. Hallownest tried tricks, rituals, and prayers, and even the Five Great Knights had no answer to a formless enemy. A higher being who feeds on belief cannot be fought with a Nail. Our lore guide traces that rivalry from the start.
The Infection is not just backstory; it visibly worsens during your playthrough at a scripted point. The trigger is tied to your own progress.
Per the wiki, after the Knight kills one of the 
Dreamers or acquires the Monarch Wings, the Infection from the Black Egg spreads and intensifies. The 
Forgotten Crossroads become the Infected Crossroads.
According to the wiki, creatures there become "mutated, bloated, and dripping with Infection," blobs of it block paths, and a sharp sickly smell hangs in the air. Quiet enemies turn into far more dangerous infected forms.
Infected bugs lose themselves by degrees, and the late stages are where they become genuinely reckless. The plague trades sanity for aggression.
Per the wiki, at advanced stages infected bugs "may even lose their sense of self-preservation," as seen with Belflies that explode and Violent Husks that charge without fear. The husks that fill Hallownest, like the Husk Miners, were once ordinary citizens.
The clearest single example is 
Myla, a friendly miner in the Crossroads who slowly succumbs across your visits. Watching her songs turn to broken muttering is the game showing the Infection at a personal scale.
There are only two ways to actually stop the Infection, and both go through the Radiance herself. Containing the symptoms is not the same as curing it.
The Pale King's answer was containment: seal the Radiance inside a mindless Vessel, the 
Hollow Knight, held shut by the three Dreamers. Per the wiki this only dams the plague, since the Vessel was not truly empty and the Infection leaks through.
The real cure is destruction. Claiming the Void Heart and killing the Radiance in the dream ends the Infection at its source, the outcome our endings guide calls Dream No More. The Godmaster route goes further, uniting the void under the 
Shade Lord to erase her entirely.
The Infection ties the Radiance, the Dreamers, and the endings together. These guides follow each strand to its root.
Game data and screenshots adapted from hollowknight.fandom.com, used under CC BY-SA 3.0. Original content remains the property of the wiki contributors and Team Cherry.