Covid-19 brain fog, conceptual illustration. Brain fog describes a condition where someone experiences short-term memory loss, confusion and an inability to concentrate. Covid-19 is a respiratory disease caused by the new coronavirus SARS-Cov-2, which emerged in Wuhan, China, in December 2019 and quickly spread to become a pandemic. Some individuals who have recovered from the initial respiratory disease find that they develop brain fog. The cause is not known and there doesn't appear to be any correlation with the severity of the initial disease or underlying medical conditions.