Latest study in the translational biomedical research programme initiated by Invest in ME Research and focusing on the role of infection and the immune system in ME/CFS including autoimmunity. Enrollment by invitation only (Severe Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) patients will be identified by Epsom and St Helier CFS Service and the Clinical Research Network Eastern.) Household controls will be recruited through patients participating in the study. Brief Title: Autoimmunity in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) Official Title: Defining Autoimmune Aspects of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) Brief Description: This study investigates whether there is an immune response directed against gut microbes or food and parts of the body in severe Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) patients. Main ID: NCT03254823 Principal Investigator: Simon Carding Primary Sponsor: University of East Anglia Secondary Sponsors: Quadram Institute and Invest in ME Research Estimated Completion Date: 30 September 2020 Current Primary Outcome Measures: Measurement of antibody levels [ Time Frame: 3 years ] Measurement of serum antibodies that are reactive with intestinal microbes or foods. Antibody levels will reflect the presence of an immune response directed against gut microbes in severe ME/CFS patients. Measurement of cellular immune responses [ Time Frame: 3 years ] Measurement of memory T and memory B cells (from peripheral blood mononuclear cells in the blood) specific to gut microbes (from stool sample). The presence of memory T and B cells will indicate a systemic (whole body) immune response directed against gut microbes in severe ME/CFS patients. Expression of microbial and viral DNA/RNA [ Time Frame: 3 years ] To identify the bacterial, fungal and virus populations in stool samples. Current Secondary Outcome Measures: Measurement of autoimmunity [ Time Frame: 3 years ] Measurement of serum antibodies and immune cells that are reactive with human cells, such as cells of the central nervous system. The presence of an immune response against human cells and human cell proteins is indicative of the presence of autoimmunity. Links to study details: WHO International Clinical Trials Registry Platform: http://apps.who.int/trialsearch/Trial2.aspx?TrialID=NCT03254823 NIH Clinical Trials.gov: https://clinicaltrials.gov/ct2/show/record/NCT03254823 NHS choices: http://www.nhs.uk/Conditions/Chronic-fatigue-syndrome/Pages/clinical-trial-details.aspx?TrialId=NCT03254823&Condition=Chronic fatigue syndrome&pn=1&Rec=0&CT=0 Other links: Quadram Institute: https://quadram.ac.uk/targets/me-cfs/ Invest in ME Research: http://www.investinme.org/index.shtml .