It sounds sooooo good and original.
(from the wiki)
In the future, Earth has been colonized by the "Souls", a parasitic alien race which inhabit human bodies, erasing the original occupants. Melanie Stryder, is one of the last free humans alive. On a raid to find food, she meets Jared Howe, another human and the two fall in love. Melanie is chased by a group of Souls, intent on capturing rogue humans. To avoid capture she throws herself down an elevator shaft nearly killing herself. A Soul named Wanderer is inserted into Melanie's body with the hopes of accessing her memories and finding the other humans in her group. Melanie remains partially conscious and bombards Wanderer with her memories of Jared and her brother Jamie. Wanderer begins to form an emotional connection to both and heads into the desert to find their hideout.
Near death from dehydration, Wanderer is found by Melanie's uncle Jeb and taken to a series of desert caves that serve as a hideout for dozens of free humans. Her presence angers many of the residents, who view her as a parasite, but Jeb refuses to allow her to be killed. Eventually, after helping out and showing kindness, Wanderer begins to be accepted by the group, who call her Wanda. She must deal with her increasing attraction to Ian O'Shea, one of the residents and Melanie's continued feelings for Jared. She becomes angry when she discovers that the humans are experimenting with ways to remove Souls from their human hosts, but that the procedure has so far proven fatal for both parties.
When Jamie becomes sick, Wanda uses her identity as a Soul to gain access to a hospital and procure the needed medicine, earning her more trust. However, Wanderer's old seeker tracks her to the caves and kills one of the inhabitants before being captured. Remorseful, Wanda reveals a method for safely extracting Souls from their host body and the humans use it on several of their loved ones. Wanda asks to be removed from Melanie's body so that Melanie can live again, opting to die rather than possess another human. The process is a success; however, her friends go against her wishes and implant her in the body of a human girl whose mind had been left blank after a Soul was extracted. Melanie and Wanda begin romantic relationships with Jared and Ian respectively. At the end of the book, the group runs into another group of surviving humans, which includes another turned soul.