The Tree of the Knowledge of Good and Evil was one of two trees situated in the middle of the Garden of Eden, the other being the Tree of Life. God had commanded Adam the first man,to not eat the fruit of this one tree. Satan, in the form of a serpent, lied to Eve (who knew of the command), tempting her to disobey the command. This led she and her husband Adam to disobey God.
The tree itself and its fruit it are not described. From the account, it was significantly different as to have an aroma and appearance to risk everything try it. Like all trees, it had been planted and grown to fruition by God Himself during the first week of The Creation. It stood tall enough for its fruit to be picked, drawing nutrients from a root system that allowed it to stand firm.
Created mature and "good", there was nothing particularly bad about the fruit of the tree. In the aftermath of the feast upon the tree, there does not seem to have been any physical distress. It is the name that distinguishes the tree: it would provide a forbidden knowledge. The transgression that followed had been avoidable by simply obeying God. The agreement God had made with Adam had been a test of his faith in God. With his wife, he had failed the test.