The degree of CFS/ME severity can differ widely among patients, and will also vary over time for the same patients. Severity can vary between getting unusually fatigued following stressful events, to being totally bedridden and completely disabled. The symptoms will tend to wax and wane over time. This variation, in addition to the fact that the cause of the disease is not yet known, makes this illness difficult to diagnose.

"They are as impaired as people with multiple sclerosis or AIDS or who are undergoing chemotherapy for cancer," said Dr. William C. Reeves of the CDC. "They don't die, but they are severely debilitated.¹ "

Mild Cases
The person would normally be able to wash, dress, bath, use the toilet and get up and down stairs without difficulty. The ability to plan a meal is unlikely to be impaired and the tasks involved in preparing and cooking food are unlikely to be restricted for the majority of the time. The ability to walk long distances may be reduced, but the person is likely to be able to walk short distances on an unrestricted basis most of the time. Their judgment, thought processes and means of communicating, are usually not affected to the extent that they would be unable to find their way around in familiar and unfamiliar places. While some people may be able to continue with some work/education, they are likely to have stopped all leisure and social pursuits, often needing to take days off. Most will use the weekend to rest in order to cope with the week.

Moderate Cases
Those with a moderate level of disability will have reduced mobility and are restricted in all activities of daily living. They would be expected to be able to manage some personal care/hygiene and preparation of food/drink without help from another for some, or most, of the time. Tasks will often/usually take longer than normal, and be fragmented throughout the day/week. Tasks will be followed by a period of rest. Levels of fatigue and symptom severity may vary during the day, or from day to day, this will also depend on whether they are classed as mild/moderate or moderate/severe. They will experience some degree of cognitive dysfunction, and for some this could be significant. They have usually stopped work/education and require rest periods, often sleeping in the afternoon for one or two hours. Sleep quality at night is generally poor and disturbed. The ability to walk more than a hundred feet consistently is likely to be restricted in moderate cases.

Severe Cases
Those with a severe/very severe level of disability will only be able at most to carry out minimal daily tasks eg. face washing and brushing teeth, and will need help with all personal care and preparation of food/drinks. They will spend most of the day house/bed bound or otherwise immobile. They will be wheelchair dependent due to severe restriction in their ability to walk, there may be clinically evident muscle wasting. They may need aids such as a hoist and stair lift. There is a requirement for supervision at home and out of doors, due to the significant impairment of both physical and cognitive functioning. Severe sufferers require 24-hour care, particularly due to the sufferers' significantly disturbed sleep pattern. They require general care both day and night; preparing meals/drinks, medication and using the toilet. Symptom control is difficult as CFS/ME sufferers often have new sensitivities to medications. They are usually unable to tolerate noise and are extremely sensitive to light. They are also sensitive to touch and chemicals/smells.

Many severe sufferers of ME/CFS may be much more disabled and dependent than that described above. Many sufferers, of all severity levels, experience pain, often severe, often unrelenting, and not always alleviated by analgesics. Some sufferers also have other intrusive symptoms eg. blackouts, paresthesia, stroke-like symptoms etc.