https://www.psychologytoday.com/us/blog/urban-survival/201701/new-research-shows-depression-linked-inflammation
Previous studies have linked depression with higher level of inflammatory markers compared to people who are not depressed. When people are given proinflammatory cytokines, people experience more symptoms of depression and anxiety. Chronically higher levels of inflammation due to medical illnesses are also associated with higher rates of depression. Even brain imaging of people with depression show that their brain scans have increased neuroinflammation. When your body is in an inflammatory state fighting off the common cold or flu, you can experience symptoms overlapping with depression— disrupted sleep, depressed mood, fatigue, foggy-headedness, and impaired concentration.