"I don't even talk about it as a psychiatric condition any more. It does involve psychology, but it also involves equal parts of biology and physical health," he said.
    
Research has shown that healthy people can be temporarily put into a depressed, anxious state when given a vaccine that causes a spike in inflammation.
    
A family of proteins called cytokines sets off inflammation in the body, and switches the brain into sickness mode.
    
Both cytokines and inflammation have been shown to rocket during depressive episodes, and - in people with bipolar disorder - to drop off in periods of remission.
    
Another researcher Turhan Canli of Stony Brook University in New York believes infections are the most likely culprit behind inflammation and goes as far as to say that we should rebrand depression as an infectious - but not contagious - disease.
    
However, infection is not the only way to set off inflammation. A diet rich in trans fats and sugar has been shown to promote inflammation while obesity is another risk factor.
    
Carmine Pariante, a psychiatrist at Kings College London, said that in between five to 10 years there may be a blood test that can measure inflammation in people with depression so that they can be treated accordingly.

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