Taint is the purple biomass you will find outside of your bio-dome. Taint can be removed with Ethereal Bloom, Wand Focus : Purify, Silverleaf splash potion, Totem of Dawn, or the Dawn Machine. The Taint will regrow over areas that are left unprotected afterwards. Taint will spread to anything and will cover it with Fibrous Taint. Some blocks, such as dirt or wood, can turn into Tainted soil or Crusted Taint, respectively. Taint gives you the Taint Poison effect if you stand on it for too long. Taint reacts differently with mobs though as a sheep will turn into a hideous Tainted Sheep which is slightly faster and has more health. Tainted mobs are also aggressive. Any mob which does not have a tainted form will turn into a purple slime or simply die. There are other mobs which are spawned from taint such as the Taintacle and Spore Swarmer. Silverwood trees create a barrier which prevents Taint from getting too close. You could plant some of these to make a Taint-free base.