Alzheimer’s Disease (AD): The purpose of this study is to look at whether BIIB080 (the study drug) can help people with Alzheimer’s Disease (AD), how safe it is, and how the body handles taking it (known as tolerability). Participants may be eligible to take part in the study if they have mild cognitive impairment due to Alzheimer’s Disease (AD), mild AD or dementia. AD is a type of dementia in which abnormal proteins, called beta-amyloid and tau, accumulate in the brain and damage nerve cells. People with AD often have trouble thinking, speaking, remembering things, and performing their daily activities. This condition is common in older people and gets worse over time. Only people between 50 and 80 years of age who are diagnosed with AD can take part in the study.