Mechanisms involved
This condition is explained through the mechanisms below. Each one is taught as ordinary physiology first.
Primary
Secondary / contributing
ME/CFS
The energy envelope, and what a crash looks like
- PEM is delayed — the crash usually lands 12–48 hours after the effort, which is why it's so easy to blame the wrong day.
- Boom-and-bust: good days get spent, and the recovery costs more than the day was worth.
- Heart-rate ceilings from a wearable are a practical way to find the edge of the envelope before you feel it.
Illustrative pattern, not real data. The defining feature of ME/CFS is post-exertional malaise: a delayed, disproportionate crash after physical or cognitive effort. Staying inside the envelope (pacing) keeps the baseline flat and slowly rising; repeatedly exceeding it produces the boom-and-bust pattern and can lower baseline over time. This is why graded exercise therapy was removed from NICE guidance in 2021.
Immune cells involvedWho is actually in the tissue
The infection resolves; the immune response often does not.
Selected cell
T cell
Directs the tone of the response. A shift toward Th2 and Th17 signalling keeps tissue inflamed long after the original trigger has gone.
Persistent T cell activation and altered exhaustion markers are seen months after the acute illness.
Schematic drawings, not micrographs. Immune findings here describe mechanism and association; none of them are diagnostic tests.
What it is
Symptoms persisting beyond twelve weeks after SARS-CoV-2 infection, most commonly fatigue, post-exertional malaise, orthostatic intolerance, cognitive difficulty and pain. It is not one disease; it is a set of post-viral phenotypes that need to be distinguished before treatment.
How the mechanisms produce this
- Small-fiber and autonomic involvement explains the orthostatic phenotype, which meets POTS criteria in a substantial minority.
- Persistent immune activation, including mast-cell involvement, accounts for the flushing, gut and reactivity symptoms.
- Where post-exertional malaise is present, the picture overlaps ME/CFS and graded exertion can cause lasting harm.
- Sleep disruption and sustained inflammatory signalling lower pain thresholds on top of everything else.
What tends to help
- Screen for post-exertional malaise first — it changes every subsequent recommendation.
- Active stand test or tilt to identify the orthostatic phenotype.
- Pacing inside an energy envelope, with autonomic reconditioning only where PEM is absent.
- Treat identifiable co-travellers: iron, thyroid, sleep apnoea, mast-cell reactivity.
Where the evidence is thin
Whether viral persistence, autoimmunity, microclotting or nerve injury dominates remains unresolved, and probably differs by person.