Think of Taint as a virus. Once it starts infecting the world, it will continue to grow and slowly take over more and more of the world. It will infect creatures and will turn any block into taint.

Taint is spread by Giant Taint Seeds. Once a Giant Taint Seed spawns, the area around it will quickly become tainted, with most blocks converting to tainted blocks (many modded blocks are unaffected). When the Giant Taint Seed is killed, the taint will quickly die off. You can also harvest the tainted dirt or cobblestone, this is an easy way to get Vitium. At this point, dirt blocks will be restored completely, tainted trees will melt into purple liquid and evaporate over time and stone/stone type blocks will convert to porous stone, which can be mined for a wide variety of resources.