As a medic I was trained to think about the body chemistry of health and disease. It is now widely known that the body’s immune system and the body’s brain talk to each other chemically about what they are doing. When the immune system is very active defending the body against germs, it tells the brain to stop exploring and to have a rest instead. “Go to bed,” it says. “You’re ill! I need the energy down here.”

I came to recognise as a medical student how feeling ill is different to having a disease.  People can have a disease but not feel ill, and conversely they can feel ill even when there is no active disease process. People can feel ill from unease. Sometimes that unease arises when the immune system has started free-wheeling in ever-increasing activity, like a communication system that has got stuck in a screaming whistle of feedback. “Go to bed!” can thus become a system-illness, not a disease-illness. This sort of illness is physical, but process-physical, not disease physical. People cannot ‘snap out of it’ by mental imagination or willpower alone. It is not ‘all in the mind’.

This sort of immune system feedback-illness presents as fatigue. The person feels their energy has gone, because it has been switched off by deep internal communication overload.

Anyone who has been bereaved knows that grieving likewise brings on a deep fatigue. It can feel as if some personal presence beyond you is torn away from every cell of the body. Grieving is physical in its adjustments. People commonly have to go to bed to mourn effectively.

Fatigue and grieving are more than parallels. They are twin processes.