Health

...the brain produces high levels of cytokines – chemicals that activate the immune system – that are released into the body. When these cytokines travel to muscles, they activate a series of chemical reactions that disrupts the ability of the powerhouse of cells – mitochondria – to produce energy.

...[T]his pathway directly causes fatigue.
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Most interesting of all, their discovery hints at a possible new treatment pathway. Antibodies that neutralize the cytokines in question are being tested in animals.

In addition, preparations are under way for clinical tests to try and sort out the possible roles in long covid of cytokines and lingering viral particles.