Inclusion Criteria:  Individuals between the ages of 55-80; MCI or moderate dementia due to AD as defined by the 2011 NIA-AA Workgroup recommendations; MoCA ≥ 8 at screening; Global CDR between 0.5-2 (inclusive) at screening; Amyloid and/or tau biomarkers indicative of AD pathology; Education level, English language skills and literacy indicates subject will be able to complete all assessments; Has a study partner who, in the investigator's judgement, has frequent, direct contact with the participant at least several days a week, can accompany the participant to all visits, and is also able to provide information to study investigator/staff; Willing and able to complete all assessment and study procedures, including blood and lumbar punctures, and clinical assessments; If on cholinesterase inhibitor and/or memantine, doses are stable for 3 months prior to baseline; Negative test results for HIV antibody and Tuberculosis (QuantiFERON) at screening; No prior BCG exposure either through birth vaccinations (born in North American) or BCG bladder cancer treatment.  Exclusion Criteria:  History of chronic infectious disease, such as HIV or untreated or active hepatitis; History of tuberculosis, positive interferon-gamma release assay (IGRA, also known as the QuantiFERON-TB test), including a test with a high reactivity to mycobacteria of non-tuberculosis variety; Prior BCG vaccination, positive T-spot tuberculosis test or a T-spot test showing significant Mycobacteria exposure; A positive SARS-CoV-2 PCR result within 3 months of screening, or known close contact with a confirmed COVID-19 positive person or symptoms highly suspicious for COVID-19 (per CDC guidelines) within 1 month of screening, including fever, cough, shortness of breath, chills, muscle pain, new loss of taste or smell, vomiting or diarrhea, and/or sore throat, based on clinician's judgment; History of treatment with metformin within the past one year; Treatment with other investigational agents which, at the discretion of the investigator, interfere with safety and/or study outcomes; Current treatment with immunosuppressants (calcineurin inhibitors, corticosteroids, or biological or cytotoxic immunosuppressants, or disease or condition likely to require high dose steroid or immunosuppressive therapy); Other conditions or treatments associated with increased risk of infections or treatment with immunosuppressive medications for any reason; Current treatment with aspirin > 160 mg/day or chronic, daily NSAIDs; Current (as of time of study screening) or chronic use of antibiotics; History of keloid formation; Living with someone who is immunosuppressed and/or at high risk for infectious diseases (for example, HIV+ or taking immunosuppressive medications for any reason), or in a job (e.g. healthcare) in which the subject works with immunosuppressed populations; Other/confounding neurological or psychiatric condition, unstable medical or psychiatric conditions, contraindications to BCG use and lab abnormalities or concurrent medication use posing risk for BCG or study procedures; Laboratory abnormalities in B12, Folate, TSH, or other common laboratory parameters that may contribute to cognitive dysfunction per clinician judgment; Laboratory abnormalities in CBC, electrolytes, LFTs, BUN, Cr, total serum immunoglobulins, ESR, CRP, or urinalysis posing risk to treatment with BCG per clinician judgment; Laboratory abnormalities in PT-INR, which would pose a risk to performing the lumbar puncture procedure; Discontinuation of cholinesterase inhibitor or memantine within one month (28 days) prior to baseline visit; Females who are pregnant, lactating or of child-bearing potential; If male with female partner(s) of childbearing potential, unwilling or unable to adhere to contraception requirements specified in the protocol. Administration of live vaccine within 30 days of screening visit or BCG immunizations Administration of non-live vaccine within 14 days of screening visit or BCG immunizations If participating in optional MRI: Existing contraindication to MRI per MGH Athinoula A. Martinos Center research guidelines