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 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 itself remains pure, but in order to reach it one must touch 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.
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. Interestingly, this physical rotting and madness are not dissimilar from the effects of the True Power on Ishamael, its only known long-time user. 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.
On Saban 22, 1000 NE, Rand al'Thor and Nynaeve al'Meara used the Choedan Kal to cleanse saidin. The Asha'man and the Aes Sedai who have linked with them say the effort was successful. 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.