The Dark One tainted saidin as a counterstroke to the acts of Lews Therin Telamon and the Hundred Companions. In their daring attack, the Bore was sealed using saidin, but because saidin was touched to the Dark One in closing the gap, he was able to inflict a taint on it. The taint placed on saidin instantly drove all members of the Hundred Companions mad, and doomed all other present and future male channelers to insanity and slow death. All the ancient male Aes Sedai eventually went mad and tore the world apart, causing the Breaking and ending the Age of Legends.
The taint is described as being like an "oily slick" surrounding the "clear water" that is saidin. Saidin in and of itself remains pure, but in order to reach it one must mentally reach through the taint. The first time Rand attempted to seize saidin he "felt the taint inside of him" and it nearly made him sick.
The Dark One is able to protect male channelers from the taint through a connection that is visible in places like Tel'aran'rhiod. This connection can be cut, as happened when Rand severed Asmodean's connection in Rhuidean. Through Rand's visions in the glass columns, it was revealed that the Aes Sedai of the Age of Legends had the knowledge and ability to somehow filter out the taint from saidin, an act apparently fatal for the channelers involved, as male channelers with assistance from the female Aes Sedai fashioned the Eye of the World with pure untainted saidin.
It is speculated that the taint may be responsible for the corruption of The Ways, since saidin was used in their fashioning.
Though saidin itself is noted as being infinite in its nature, the taint apparently was not, as it was able to be removed in its entirety by Rand al'Thor after only a few hours of work by another finite energy source; the taint upon Shadar Logoth
A slow degradation of body and mind, the taint sets in some time after a man starts channeling. Those affected go mad and slowly rot away from the inside, often destroying their surroundings and loved ones in the process. This has led the remaining female Aes Sedai to hunt down and gentle all men who can channel, to prevent a second Breaking of the World.
In addition, some Aes Sedai and at least one of the Forsaken (Semirhage) believe that men will hear voices as they go mad, although it is still debated whether these are "actual" voices from past lives, or just symptoms of the madness' progression. To an extremely skilled healer, this madness appears to physically manifest during a Delving as a black, weblike construct surrounding the brain of the inflicted individual. It has been compared by Nynaeve al'Meara to the way a Compulsion manifests itself, and can be removed in a similar fashion if one possesses sufficient skill in Healing.
On Saban 22, 1000 NE, Rand al'Thor and Nynaeve al'Meara used the Choedan Kal to cleanse saidin. During the cleansing, Cadsuane and several other channelers, male and female, linked to defend Rand and Nynaeve from multiple Forsaken who Traveled to Shadar Logoth to stop them. The Asha'man and the Aes Sedai who have linked with them confirm the cleansing was successful in removing the taint, however the act did not cure those channelers already touched with madness.