I am so sorry your granddaughter has been diagnosed with ME. Unfortunately there is as yet no test for ME. It is diagnosed by symptoms and ruling out other causes for these symptoms. ME is now recognized in the governments NICE guidelines as a physical illness often triggered by a virus. Though most people recover from viruses, a small number of people remain chronically ill. Corona virus is a virus that can have this affect and people who have not recovered have been described as having long covid.. A huge study at Edinburgh University has recently shown that people who develop ME have a genetic susceptibility. Many more women and girls have this susceptibility than men and boys..
The International Consensus Criteria for ME provides the best description of symptoms. Because of the misleading name CFS, people tend to think that the main symptom is fatigue. They do not realize that people with ME can have aching muscles, headaches, light and noise sensitivity, disturbed sleep, difficulty concentrating, sore throats, nausea, dizziness and many other unpleasant symptoms. It has been described as the flu that never goes away. All these symptoms are made worse by activity, whether it is physical, emotional or cognitive. Sometimes the reaction can be delayed.
Unfortunately there is no medical cure at the moment. But the body has great healing power. There is every chance that your granddaughter will recover but it can take time. The key thing is to keep activity at a level that will not make her symptoms worse. Her recovery is the important thing now. Not pushing herself. She needs to listen to her own body and her family need to believe what she is saying about how she is feeling.
Unfortunately for many years a group of doctors, many of them psychiatrists, claimed that people with ME had had a virus but it had cleared up. They had however developed a phobia about exercise and were just unfit and could be cured by exercise, starting small but continually increasing it even as they became more and more unwell. This was disastrous and many people became extremely ill who might have recovered. Some young people were even said to want to be ill in order to avoid school or escape from life. Parents who protested that this was not their child were blamed for encouraging them to be ill. It is important to be aware of this as these views still linger in parts of the medical establishment.
The ME Association is an excellent patients support group. The Nightingale Continuum is a good face book page.
Best wishes to your granddaughter and to you all.. I know it is hard. But the love and support of your family and your belief in her, will be the greatest strength she can have behind her at the moment.