Depression, bipolar disorder, and other mental health problems arise when something goes wrong with brain function. What causes that malfunction is an open question. A new study from Denmark suggests that a serious infection or autoimmune disease could trigger a mood disorder. How might an infection or autoimmune disorder lead to a mood or other mental health disorder? Infection causes localized and body-wide inflammation. Inflammation generates substances called cytokines that have been shown to change how brain cells communicate. In autoimmune diseases, the body’s defense system attacks healthy tissues rather than threatening invaders. It’s possible that in some cases the wayward immune reaction could target brain cells and other nerve cells throughout the body.