This study is about testing a new medicine called RO7126209 for people with early to moderate Alzheimer's disease (AD). Alzheimer's is a brain disease that affects memory and thinking skills. The medicine is given through an intravenous (IV) infusion, which means it goes directly into the bloodstream using a needle. The study aims to check if the medicine is safe and how well it works.

Participants need to have a positive amyloid PET scan, which is a brain scan that helps identify Alzheimer's. They also need someone, called a "study partner," to help them with the study. The study checks your health through tests like MRIs (a type of scan that takes pictures of the inside of your body) and lumbar punctures (a test that collects spinal fluid).

• Participants will be in the study for a long time and will have to visit the study site multiple times.
• They must not join any other study during this time or donate blood.
• People with other major health problems may not be eligible to join.

Joining will help researchers learn more about Alzheimer's and develop better treatments.