What is the main conflict in Delirium by Lauren Oliver?

In Delirium, seventeen-year-old Lena is waiting for her birthday, when she can have a procedure to stop her from falling in love; a condition that has been ruled a disease. Before she can get the procedure though, she falls in love with an Invalid, Alex, one of the rebels living unaltered in the Wilds outside civilized cities. Lena discovers that much of her life is a lie, including the supposed suicide of her mother, and runs off to the Wilds, but in the process, Alex sacrifices himself for her.