Alzheimer’s Disease (AD) is a chronic progressive neurological condition, clinically characterized by memory deficits, cognitive and physical impairment, and personality changes. Traditionally, AD was considered a type of protein folding disorder. Here, the concept of AD as an autoimmune disease of the innate immune system was developed. After exploring evolutionary connections between the AD peptide β-amyloid (Aβ) and known antimicrobial peptides (AMPs), and elucidating the structural similarities between Aβ and AMPs, a mechanism of action for Aβ’s antimicrobial activity is proposed that is based on the compromise of bacterial membranes. Following these theoretical considerations, experimental evidence is presented for the production of Aβ by cells in response to infection, and for Aβ’s antibacterial and antiviral activity. Rooted in similarities of the cell membranes of neuronal and bacterial cells in terms of lipid composition and transmembrane potential, it is hypothesised that Aβ’s neurotoxicity is caused by its misguided attack on neurons as an AMP. In reversing the concept of Aβ as an AMP, the similarity of AMPs to Aβ is demonstrated in experiments revealing the neurotoxicity of two AMPs, LL 37, and cecropin A. To determine a mechanism for the progressive nature of AD, it was shown that, although apoptosis may be involved in AD, it is actually necrosis that is responsible for the propagation of neuronal cell death so characteristic of AD. With the Vicious Cycle of AD, a scheme was devised, integrating the results obtained here with data and research from other groups, which explains the chronic and progressive nature of AD as a result of Aβ’s physiological role as an AMP and innate immune system effector. Borne from Aβ’s activity as an AMP and its central role in the Vicious Cycle of AD, a question was investigated: do antibiotics, such as penicillin, that cause release of bacterial endotoxins due to their mechanism of action, trigger the Vicious Cycle of AD and thus lead to the development of AD? Preliminary evidence supporting this notion was presented.