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how could a viral infection mimic moderately severe depression? I reflected on my own symptoms during a recent bout of the flu: low mood, anhedonia, increased sleep, fatigue and a complete loss of appetite. In fact, most of the symptoms one experiences during the acute stages of an infection seem more ‘mental’ than physical. Whether you have Covid, cholera or the common cold, ‘sickness behaviour’ – how we feel and act when fighting an infection – is universal. It seems that your immune system is recruiting your brain to produce behaviours that aid your defence: primarily, wrapping up warm and staying in bed. Indeed, an increasingly accepted hypothesis is that sickness behaviour provides a survival advantage both for the individual – in saving energy to fight off the infection – and also for the herd at large: a state of mind that makes you feel low, withdrawn and tired is one that will restrain you from heading out to the party and sharing your virus with your tribe of revellers. But there was something else in Emma’s story that stayed with me. It was a brief, throwaway comment she had made in our first session, when I was exploring her previous relapses of depression: ‘It’s a bit weird, but whenever my depression comes back, or whenever I’m stressed, my eczema flares up.’ She rolled up the sleeve of her jumper to reveal a raw, red forearm. It was here, in a fairly unremarkable case study of someone undergoing CBT for depression, that I had my first glimpse of a ‘bidirectional’ relationship between the immune system and the mind. Emma’s immune system had recruited her brain – more specifically her thinking, feeling mind – to help deal with a viral infection, but her mind had also activated her immune system, causing skin inflammation. I was peering behind the curtain of my medical training: a relationship between the immune system and mental health was completely absent from all of my medical school textbooks.
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Monty Lyman (The Immune Mind: The Hidden Dialogue Between Your Brain and Immune System)