Delirium affects mainly attention and awareness, and its onset is usually sudden with a finite duration. Dementia affects mainly memory and cognitive function, with a slow and gradual onset and conditions worsening as time goes on. Delirium appeared to be the most common clinical manifestation of the SARS-CoV-2 brain infection. Patients with underlying neurodegenerative processes could be at greater risk of developing COVID-19 with neurological complications such as delirium. In this review, hallmark pathophysiological mechanisms of infection from SARS-CoV-2, such as endothelial damage, blood-brain barrier dysfunction, and local inflammation with activation of microglia and astrocytes, are used to explore the link between cognitive impairment and cognitive decline. The authors explore the damage caused by SARS-CoV-2 in the brain at a molecular level and the genetic factors that make patients more sensitive to infection. Common features reminiscent of patients with dementia or delirium are then highlighted and put in comparison to the physiological response and damages induced by viral infection by SARS-CoV-2.