**Umar H. Iqbal, Emma Zeng and Giulio M. Pasinetti \***

Department of Neurology, Icahn School of Medicine at Mount Sinai, New York, NY 10029, USA; umar.iqbal@mssm.edu (U.H.I.); emma.zeng@mssm.edu (E.Z.)

**\*** Correspondence: giulio.pasinetti@mssm.edu; Tel.: +1-212-241-7938

Received: 29 May 2020; Accepted: 10 July 2020; Published: 12 July 2020

**Abstract:** The aggregation and accumulation of amyloid-β plaques and tau proteins in the brain have been central characteristics in the pathophysiology of Alzheimer's disease (AD), making them the focus of most of the research exploring potential therapeutics for this neurodegenerative disease. With success in interventions aimed at depleting amyloid-β peptides being limited at best, a greater understanding of the physiological role of amyloid-β peptides is needed. The development of amyloid-β plaques has been determined to occur 10–20 years prior to AD symptom manifestation, hence earlier interventions might be necessary to address presymptomatic AD. Furthermore, recent studies have suggested that amyloid-β peptides may play a role in innate immunity as an antimicrobial peptide. These findings, coupled with the evidence of pathogens such as viruses and bacteria in AD brains, suggests that the buildup of amyloid-β plaques could be a response to the presence of viruses and bacteria. This has led to the foundation of the antimicrobial hypothesis for AD. The present review will highlight the current understanding of amyloid-β, and the role of bacteria and viruses in AD, and will also explore the therapeutic potential of antimicrobial and antiviral drugs in Alzheimer's disease.

**Keywords:** Alzheimer's disease; amyloid-β; antimicrobial; antiviral; antimicrobial peptide
