This is a Phase I, randomized, placebo-controlled, double-blind study to evaluate the safety, tolerability, pharmacokinetics, and preliminary activity of RO7105705 in two participant populations: healthy participants and participants with mild-to-moderate Alzheimer's disease. This study is a single dose, dose-escalation, and multiple dose study comprising approximately six single dose cohorts in healthy participants administered RO7105705, either intravenously (IV) or subcutaneously (SC), and comprising one or more multiple dose cohorts in healthy participants administered RO7105705 IV every week (QW), a total of 4 doses, and one or more multiple dose cohorts in participants with Alzheimer's disease administered RO7105705 IV QW, a total of 4 doses.