Comment by quenched

I agree it's hard to speculate: perhaps malnutrition and cardiac failure; maybe sepsis. Here are some relevant links —

Life-Threatening Malnutrition in Very Severe ME/CFS (2021)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8070213/

Caring for the Patient with Severe or Very Severe Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (2021)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8544443/

There are not many mortality studies and few ME patients have had comprehensive autopsies —

Mortality in patients with myalgic encephalomyelitis and chronic fatigue syndrome (2016)

https://www.tandfonline.com/doi/full/10.1080/21641846.2016.1...

Causes of Death Among Patients With Chronic Fatigue Syndrome (2007)

https://www.tandfonline.com/doi/abs/10.1080/0739933060080376...

Somewhat harrowing reading is Whitney Dafoe's account of becoming extremely severe —

Extremely Severe ME/CFS—A Personal Account (2021)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8145314/