A Study of LY3372993 in Healthy Participants and Participants With Alzheimer's Disease (AD)

Inclusion Criteria : Part A: * Overtly healthy males or females as determined by medical history and physical examination * Are between 18 to 45 years old, inclusive * Male participants agree to use a reliable method of birth control during the study and 3 months following the last dose of the investigational product * Female participants not of child-bearing potential * Have a body mass index of 18.0 to 32 kilograms per square meter (kg/m²) inclusive Part B: * Present with mild cognitive impairment (MCI) due to Alzheimer's disease (AD) or mild-to-moderate AD * Positive florbetapir scan * Men or nonfertile women, at least 55 years of age. Nonfertile is defined as hysterectomy and/or bilateral oophorectomy, or amenorrhea for at least 1 year * Have up to 2 study partners who can provide health information related to the study about the participant. Study partner(s) will provide a separate written informed consent to participate Exclusion Criteria: * Have significant abnormalities in brain magnetic resonance imaging (MRI); or have contraindications for MRI * Have significant allergic reactions to LY3372993, or related compounds, or have significant allergies to humanized monoclonal antibodies, diphenhydramine, epinephrine, or methylprednisolone * Have clinically significant neurological or psychological illness, or other illnesses that could affect the study results Part A: * Have family history of early onset Alzheimer's Disease (AD) * Have impaired cognitive function Part B: * History of intracranial hemorrhage, cerebrovascular aneurysm or arteriovenous malformation, or carotid artery occlusion, or stroke or epilepsy * Previously dosed in any other study investigating active immunization against amyloid beta (Aβ) * Previously dosed in any other study investigating passive immunization against Aβ within the last 6 months * Have current serious or unstable illnesses