Inclusion Criteria:  Signed written and voluntary informed consent. Patient must be willing and able to provide collection for stool specimen analyses at 2 time points. Age > 18 years, male or female. Patient must be diagnosed with histologically confirmed squamous cell carcinoma of the head and neck (oral cavity, oropharynx, larynx, hypopharynx, or nasopharynx). Patients must be eligible for curative-intent treatment with either radiotherapy or concurrent chemo-radiotherapy. Overall disease stage III-IV based on 8th edition of American Joint Committee on Cancer staging system.  Exclusion Criteria:  Receipt of induction chemotherapy. Presence of distant metastases. Any previous head and neck cancer. Any other known concurrent malignant disease. Any condition that, in the opinion of the Investigator, would interfere with patient safety, or evaluation of the collected specimen and interpretation of study result.