At the start, Melissa's journal talks about her overdosing on medicine, collapsing several times, and eventually ending with Eve possessing her body. Later on you find out that she had a kidney transplant, meaning she was on immunosuppressants. Immunosuppressants weaken the body's immune system so it can't fight off and damage transplanted organs, which help explain what was wrong with her and what could've lead to Eve easily taking over.
