A Study of LY3002813 in Participants With Memory Damage Due to Alzheimer's Disease (AD) or AD

The study will evaluate the effect of LY3002813 on brain scans. The study will evaluate the safety of LY3002813 by looking at adverse events (side effects). The study will also look at the effect the body has on LY3002813. Study participants will have mild cognitive impairment (MCI) due to AD or mild to moderate AD. The study involves 3 parts. * Part A in which participants will receive a single dose of LY3002813 or placebo (no drug). * Part B in which participants will receive multiple doses of LY3002813 or placebo for 24 weeks. * Part C in which participants will receive multiple doses of LY3002813 or placebo for up to 72 weeks. Drug will be given as an intravenous infusion (injection into a vein). For Parts A, B and C, the study will last approximately 72 weeks, not including screening of approximately 56 days. The study is for research purposes only and is not intended to treat any medical condition.