Starters by Lissa Price

Sixteen-year-old Callie, having lost every family member besides her little brother when a genocide spore killed all of those who were not vaccinated, thinks she has found a way to support them by renting her body to seniors who want to be young again, but after a neurochip malfunction results in her being stuck in the life of her rich renter, she uncovers the horrible plan of her boss and must race against time to stop it.