Maxfield's enhanced ripper-werewolf virus into Elena's body, infecting her.
However, their relationship ends due to her heightened feelings for Damon.
In the season two finale, while Damon is dying from a werewolf bite, she forgives him for forcing her to drink his blood and kisses him. However, in order to convince Klaus to give his blood to Damon, Stefan had to turn into his "Ripper" self and accompany Klaus.
In season three, Elena does everything in her power to make Stefan regain his humanity, which he switched off to satisfy, and tolerate, Klaus.
She is kidnapped and experimented on by Wes Maxfield, and learns that her father experimented on vampires.
Later, when Katherine is about to die as a result of the past 500 years catching up with her, Katherine uses a spell to possess Elena.