Brain inflammation, medically known as encephalitis, can be triggered by a virus or by the body's own immune system attacking the brain. According to a review of 101 studies with more than 4,700 survivors, just over one in four has depression three months or more afterwards. Signs of inflammation in the brain also turn up in depression without encephalitis: a 2015 study from Toronto found that 20 people in a depressive episode had, on average, about 30 percent higher levels of a marker for activated immune cells in the brain than 20 healthy people. In the blood of people with depression, levels of the inflammatory messengers IL-6 and TNF-α are also raised on average. Messengers of exactly this kind cause the typical feeling of being sick during an infection: withdrawal, exhaustion, no desire to do anything. So when depression sometimes feels like the flu, there may be a physical reason. Which comes first, the inflammation or the depression, is still unclear, and it probably works both ways.
Sources
- Watson et al., Psychiatric and behavioural sequelae following encephalitis: a systematic review and meta-analysis, Brain Communications 2026: doi.org
- King's College London, press release on the review, 18 June 2026: kcl.ac.uk
- Setiawan et al., Role of translocator protein density, a marker of neuroinflammation, in the brain during major depressive episodes, JAMA Psychiatry 2015: pubmed.ncbi.nlm.nih.gov
- Dowlati et al., A meta-analysis of cytokines in major depression, Biological Psychiatry 2010: pubmed.ncbi.nlm.nih.gov
- Dantzer et al., From inflammation to sickness and depression: when the immune system subjugates the brain, Nature Reviews Neuroscience 2008: pubmed.ncbi.nlm.nih.gov
- Beurel, Toups, Nemeroff, The bidirectional relationship of depression and inflammation: double trouble, Neuron 2020: doi.org