Patients with Common variable immunodeficiency (CVID) have various forms of autoimmune and auto inflammatory disorders. The study will investigate if intervention with Rifaximin modifies the gut microbiota with a subsequent alteration in markers of systemic immune activation and inflammation in patients with CVID. The investigators hypothesize that the gut microbiota of CVID patients, at least partly through interaction with the innate immune system within the intestine, contribute to a low-grade systemic inflammation in these patients, and that an intervention with the non-absorbable antibiotic Rifaximin attenuates systemic inflammation through modulation of the gut microbiota. The study may lead to increased understanding of the interaction between microbiota and the immune system. The study could give new insight into important disease processes in relation to the interaction between the microbiota, the intestine and the systemic compartment, and potentially be the basis of new therapeutic strategies in these patients to prevent and down-regulate the auto-inflammatory and autoimmune complications seen in CVID. The findings could also be of relevance for other disorders where the interaction between microbiota and intestinal and systemic inflammation is involved such as various cardiovascular and metabolic disorders.

The investigators hypothesize that the gut microbiota of CVID patients, at least partly through interaction with the innate immune system within the intestine, contribute to a low-grade systemic inflammation in these patients, and that an intervention with the non-absorbable antibiotic Rifaximin attenuates systemic inflammation through modulation of the gut microbiota.

Inclusion Criteria:

• 18 ≥ and <75 years of age
• A diagnosis of CVID: decreased serum levels (> 2 SD) of immunoglobulin (Ig)G, IgA and/or IgM and exclusion of other forms of hypogammaglobulinemia

Exclusion Criteria:

• Previous treatment with antibiotics within the last 12 weeks
• History of hypersensitivity to Rifaximin or other Rifamycin derived antimicrobial agents, or any of the components of XIFAXAN
• Comorbidity not related to CVID- i.e. conditions or symptoms that may influence with the patient safety or compromise the study results (e.g., cardiovascular disorders, alcoholism, psychiatric disease, HIV infection etc.)].
• Polypharmacy with increased risk for interactions. i.e. patient with an extensive medication lists (e.g. 10 drugs or more) this may influence with the patient safety or compromise the study results
• Malignancy of any cause
• Impaired kidney function (i.e., estimated glomerulus filtration rate <50 ml/minute/1.73 m2]
• Impaired liver function (Alanine aminotransferase > 150 U/l) or established liver cirrhosis.
• Pregnant or planning to be pregnant in the study period to avoid interference of pregnancy with gut microbiota (not because of toxicity].
• Nursing
• On-going infection, including GI infection
• The use of probiotics for the recent 6 months
• Any immunosuppressive drugs,