The bugs of Hallownest were twisted out of shape by that ancient sickness. First they fell into deep slumber, then they awoke with broken minds, and then their bodies started to deform...
The Infection, sometimes referred to as the ancient sickness, bright light, or the plague, is the main antagonistic force in Hollow Knight, leading to the deaths of thousands of bugs and ultimately the downfall of the kingdom of Hallownest.
Arrival of the Pale King
Before the arrival of the Pale King, the Moth Tribe worshipped their creator, a higher being called the Radiance. After the kingdom of Hallownest was founded, the Moths turned their backs to the Radiance and started worshipping the Pale King instead. Over time, they ended up forgetting about the Radiance.
First Infection wave
However, the Radiance was not entirely forgotten, as memories of her still lingered. Eventually, bugs started to have dreams about the Radiance. This caused the Infection to appear for the first time. It started to manifest itself as bugs struggled against the Radiance's influence and tried to suppress it. The bugs of Hallownest tried all sorts of tricks, rituals, and prayers to stop the Infection, but none of these methods worked. Even the Five Great Knights did not know how to defeat a formless enemy such as the Infection.
The Pale King decided to use a Vessel to seal the Radiance away which would stop the Infection. These Vessels were the Pale King and the White Lady's children, born in the Abyss with Void inside their shell. The chosen Vessel would be unaffected by the Radiance's influence, and would therefore be able to seal her indefinitely. In the end, the Hollow Knight was judged to be pure and chosen to become the Vessel who would seal the Radiance.
Second Infection wave
However, the Hollow Knight turned out to be impure, tarnished by an idea instilled. Despite that, they were eventually sealed with the Radiance in the Temple of the Black Egg. Since the Hollow Knight was susceptible to the Radiance's influence, the Infection returned after some time, quickly devastating Hallownest and plunging the kingdom into ruin.
Hallownest remained in ruin for a considerable amount of time until eventually the Hollow Knight's shell started to crack, amplifying the Infection and causing corpses to be reanimated.
The Knight returned to Hallownest after a call went out from either the Radiance or the Hollow Knight. After the Knight kills one of the Dreamers or acquires the Monarch Wings, the Infection from the Black Egg spreads and intensifies. Many creatures in the Forgotten Crossroads become mutated, bloated, and dripping with Infection. Blobs of Infection block various paths and a sharp sickly smell hangs in the air.
During the Hollow Knight and Sealed Siblingsendings, the Radiance and the Infection are sealed inside the Knight. In the Dream No More ending, the Radiance is consumed by the Void, stopping her influence on the dreams of Hallownest's inhabitants and putting an end to the Infection.
If the Knight defeats the Absolute Radiance in the Pantheon of Hallownest, the Absolute Radiance is completely destroyed by Void given Focus. As a result of this, during the Embrace the Void ending, Hornet watches vines of Infection shrivel up and lose their colour, thus marking the end of the Infection.
The Infection initially manifests itself through dreams, using desires and planting false hopes in the minds of bugs. Eventually, the Infection grows dominant in their minds, which are broken, and their wills enslaved. This allows the Radiance to exert control over the bugs overtaken by the Infection and make them become part of her union. As a result, they return to primal instincts and are driven mad, becoming much more courageous and aggressive. At advanced stages, they may even lose their sense of self-preservation, as seen with Belflies and Violent Husks.
Some bugs managed to withstand the Infection. The Mantis Tribe actively resist it, which allows them to retain their intellect.Bardoon resists the Radiance's allure and went to a quiet place in Kingdom's Edge to get away from the Infected creatures whose minds were lost to the Radiance.
The Infection manifests itself physically in beings in several ways, generally depending on how far the Infection has progressed. What is true for all beings who are Infected is that they release splatters of orange fluid when struck, while non-Infected bugs release white particles. The eyes of most Infected beings glow orange, although this is not true for all of them.
In later stages of the Infection, the body of the Infected being starts to deform. Their body becomes distorted, with orange growths covering their body and sometimes their entire body swelling up with Infection. Orange gas may also seep from the growths. While Infected beings may be weak by themselves, the Infection grants them strength and empowers them.
Besides its direct effect on bugs and creatures, the Infection manifests itself physically in other ways. Corpses can be reanimated by the Infection or by Infected lifeforms such as Lightseeds or flies. It can also create single-celled lifeforms such as Infected Balloons. Lastly, the Infection can manifest as growths, vines, and noxious gas which cover the caverns.
The Infection apparently has a sweet smell and taste.
↑ 1.01.11.2Violent HuskHunter's Journal entry: "The bugs of Hallownest were twisted out of shape by that ancient sickness. First they fell into deep slumber, then they awoke with broken minds, and then their bodies started to deform..."
↑Entombed Husk Hunter's Journal entry: "Inside these shambling corpses is a bright light that pierces any darkness."
↑Radiance Hunter's Journal entry: "The plague, the infection, the madness that haunts the corpses of Hallownest..."
↑Elderbug: "Hallownest, it was once called. Supposedly the greatest kingdom there ever was, full of treasures and secrets. Hm. Now it's nothing more than a poisonous tomb full of monsters and madness."
↑ 5.05.15.25.35.4Seer: "The folk of my tribe were born from a light. Light similar to Essence, similar to that powerful blade, though much brighter still. They were content to bask in that light and honoured it... for a time. But another light appeared in our world... A wyrm that took the form of a king. How fickle my ancestors must have been. They forsook the light that spawned them. Turned their backs to it... Forgot it even. And so this kingdom was born from that betrayal. But the memories of that ancient light still lingered, hush whispers of faith... Until all of Hallownest began to dream of that forgotten light."
↑ 6.06.1Pale King: "You shall seal the blinding light that plagues their dreams."
↑ 7.07.1Moss Prophet: "Light is life, beaming, pure, brilliant. To stifle that light is to suppress nature. Nature suppressed distorts, plagues us. Embrace light! Achieve union!"
↑Slobbering Husk Hunter's Journal entry: "The infection that swept through Hallownest so long ago... they say that the harder you struggled against it, the more it consumed you."
↑Soul Master Hunter's Journal entry: "Head of the Soul Sanctum. Hoarded soul hoping to stave off Hallownest's affliction, but eventually became intoxicated by its power. The bugs of Hallownest tried all kinds of tricks and rituals and prayers to rid themselves of the infection. But to no avail! Perhaps the infection came from somewhere deep inside of them that they could not escape."
↑Dung Defender: ""Then that nasty business with the affliction… We knights defend against the physical, but a formless enemy. How to defeat such a foe?"
↑Elderbug: "A sharp sickly smell has started to drift from the well beside town."
↑Dream No Moreachievement description: "Defeat the Radiance and consume the light"
↑Soul Master: "In my dreams I could see it. The Kingdom's salvation, the cure for the plague... the answer was in the soul that animates our bodies."
↑The Hunter: "I have felt that desire. The desire to take the infection into myself. To become stronger, more powerful... these thoughts haunt my dreams during the darkest times. A false hope, but it can burn so brilliantly in one's mind."
↑ 28.028.128.228.3Furious Vengefly Hunter's Journal Entry: "Vengefly whose body has been distorted by continued infection. The infection that passes from creature to creature grants strength and courage, but it also seems to enslave the will. Would you make such a bargain?"
↑MylaDream Nail dialogue: "... how much longer... KILL IT... how much longer... DANGEROUS... how much longer... KILL IT... how much longer... KILL THE EMPTY ONE..."
↑ 30.030.130.2Bardoon: "I resist the light's allure. Union it may offer, but also a mind bereft of thought... To instinct alone a bug is reduced..."
↑White Lady: "The first is inevitable on current course, regression, all minds relinquished to that pernicious plague."
↑ 32.032.1Violent Husk Hunter's Journal entry: "Maddened by massive infection. Charges at enemies with no thought for its own preservation."
↑Quirrel: "The sickness in the air that clouds the mind of lesser beasts... they resist it. They retain their intellect and their honour, though also their lethal traditions."
↑Bardoon: "For quiet retreat did I climb up here, away from spitting creatures. Ormmph... Yes. High up. Away from simple minds, lost to light."
↑ 36.036.1Volatile Gruzzer Hunter's Journal entry: "Gruzzer swollen with infection and primed to burst. Horrible goo and noxious gas has started leaking down from above."
↑ 37.037.137.2Volatile Mosskin Hunter's Journal entry: "Plant-like bug swollen with infection. Releases deadly gas when threatened. The gas that seeps from their growths smells invitingly sweet, but it will burn through shell and flesh if you get too close!"
↑White Lady: "The Vessel may itself be weak, but it is much empowered by that force within."
↑Wandering Husk Hunter's Journal entry: "The remains of a bug, animated by a strange force. Wanders the roads where it once lived. These "civilised" bugs of Hallownest were weak in life and now they are equally weak in death."