Inclusion Criteria:  Women Age 18-70 years History of breast cancer stage I, II, or III ≥1 year post-primary cancer treatment completion English speaking Physician medical clearance Ambulates without assistance No antibiotics in past 90 days Willing to avoid taking probiotics during the study Has access to a safe place to perform aerobic exercise (e.g., willing to walk in neighborhood or has gym membership or owns relevant exercise equipment at home, etc.)  Exclusion Criteria:  Metastatic or recurrent cancer Another cancer diagnosis in the past 5 years (not including skin or cervical cancer in situ) Unstable angina New York Heart Association class II, III, or IV congestive heart failure Uncontrolled asthma Interstitial lung disease Current steroid use Told by a physician to only do exercise prescribed by a physician Dementia or organic brain syndrome Schizophrenia or active psychosis Participating in >20 minutes of exercise on ≥2 days/week in the past six months Anticipate elective surgery, medication changes or antibiotics during the study Contraindication to ≥moderate intensity aerobic exercise Pregnant or anticipate pregnancy during the study