Alzheimer's disease (AD) is a degenerative disease of the brain that is associated with dementia, brain atrophy, accumulation of hyperphosphorylated tau protein and amyloid-beta peptide in hippocampus and cortex region of the brain. The development of AD is a multifactorial process that may also involve infection with bacterial pathogens. Bacteria including spirochetes have the potential to initiate the cascade of events, leading to an inflammatory condition of the central nervous system. Bacteria and spirochetes are activators of pro-inflammatory cytokines; generate free radicals, nitric oxide and further induction of apoptosis. Infection with these microbes may be considered as a risk factor for the pathophysiology of AD or to cognitive changes. Neuroinflammation in the brain may be a reaction to the presence of bacteria. The brain is normally sealed behind specialized blood vessels that make it very difficult for things like bacteria in the blood to enter. However, at least one of the genetic risk factors for Alzheimer's disease may cause these blood vessels to lose some of their integrity, which could allow bacteria to enter and colonize the brain. The Alzheimer's brains contained different proportions of specific bacteria compared with the healthy brains.