This study will estimate the radiochemical and radiation safety and assess the efficacy of [18F]MK-3328, a novel positron emission tomography (PET) tracer. The study safety hypotheses will test whether [18F]MK-3328 is sufficiently safe and well-tolerated, based on an assessment of clinical and laboratory evaluations and adverse experiences in healthy participants, including healthy elderly (HE) participants, and Alzheimer's disease (AD) participants, to permit continued investigation. The study efficacy hypothesis will test whether [18F]MK-3328 can discriminate between AD participants and cognitively normal elderly control participants based on tracer volume of distribution, or one of its surrogates, in brain posterior cingulate gyrus.