The NIA BRAIN-ICU-2 study will define the relationship of intensive care unit-related delirium to the development of Alzheimer?s disease and related dementia (ADRD). To understand the brain tissue damage of acute delirium caused by SARS-CoV-2, we will perform new neuropathological studies of critically ill patients affected by COVID-19, as well as those with ADRD with/without critical illness. By studying COVID-19, we will better understand the relationship between delirium and ADRD.