Epigenetics is turning DNA on and off. Another description is gene expression. A 2017 article ties this to CFS and there is a lot of other literature connecting epigenetic changes to other conditions and illnesses. This article not only tied it to CFS but with methylation issues seen with CFS/ME.
We detected 12,608 differentially methylated sites between ME/CFS patients and healthy controls predominantly localized to cellular metabolism genes, some of which were also related to self-reported quality of life health scores. Among ME/CFS patients, glucocorticoid sensitivity was associated with differential methylation at 13 loci.Epigenetic modifications and glucocorticoid sensitivity in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) 
A Feb, 2019 article in Cell, they found that bacteria influences it.
Thus, the microbiota can shape the post-translational landscape of the host proteome to regulate microRNA activity, gene expression, and host development. Our findings suggest a general mechanism by which the microbiota may control host cellular functions, as well as a new role for gasotransmitters.Regulation of MicroRNA Machinery and Development by Interspecies S-Nitrosylation 
A more plain English summary is in ScienceDaily.
Additional Literature Worth Reading
- Microbial Agents as Putative Inducers of B Cell Lymphoma in Sjögren’s Syndrome through an Impaired Epigenetic Control: The State-of-The-Art. 
- Diet, Gut Microbiota, and Obesity: Links with Host Genetics and Epigenetics and Potential Applications. 
- Autoimmunity and Inflammation in CVID: a Possible Crosstalk between Immune Activation, Gut Microbiota, and Epigenetic Modifications. 
- Antibiotics suppress colon tumorigenesis through inhibition of aberrant DNA methylation in an azoxymethane and dextran sulfate sodium colitis model. 
- Epigenetics, DNA Organization, and Inflammatory Bowel Disease. 
- Folate and epigenetics: why we should not forget bacterial biosynthesis. 
- Microbiota-sensitive epigenetic signature predicts inflammation in Crohn’s disease. 
- Diet, Microbiome, and Epigenetics in the Era of Precision Medicine. 
While recent studies are showing more impact of the microbiome (gut bacteria) on health, it also impacts how DNA behaves. We know little of the connections (i.e. this strain of this bacteria will turn some specific DNA on or off is still an unknown), we can reasonably infer that you have a condition/illness and a shift in microbiome — that they are connected and further more, there is a reasonable chance that correcting these shifts will improve the illness or condition. With a 100% normalization of the microbiome — there is a reasonable chance that the condition may go into remission.
There is a chart of relationship on my site. A listing also here. And suggestions (based on reports from the literature) of how to normalize for different conditions (when information has been published) here.