The infection is symbolised by the color orange. There seem to be three prominent stages of infection: the first being the eyes of the infected bug being dotted with orange pupils, the second being stringed with orange vines and small spheres of the infection, and the third being the most serious, where the infected bug becomes completely consumed by infection, causing large, orange cysts appearing on the body of the infected bug. In the case of the Hollow Knight, large cysts appear under its cloak on its stomach, and grow larger as the battle continues.
After the Radiance was shunned and forgotten by the bugs of Hallownest, it grew angered and enraged. It started entering the minds of bugs, and eventually, almost every bug dreamed of the intense light of the Radiance. The dreams manifested into a horrible, raging light, which infected the minds of the bugs it entered. This became the infection that haunts the corpses of Hallownest.
Those under the control of the infection, and indirectly, the Radiance, lose their will, but in turn are gifted great strength and courage. Once a bug has dreamed of the light, it will fall into a deep slumber, then awake with a broken mind, and then, if the infection becomes more serious, its body will start to deform. Some said that the harder a bug struggled against its pestilence, the more it consumed said bug. The infection seems to kill the insect it infects, or, once the infection inside is destroyed, the infection somehow kills the bug there on the spot. All the bugs under the influence of the plague are connected in a hive mind and turned into a feral state, much like they were before the new monarch of Hallownest, the Pale King, arose from the deceased Wyrm.