Inclusion Criteria:  18 years or older Covid-19 infection with severe disease defined within the last 12 months (defined as one or more of the following: hospitalization, need for oxygen supply, need for intensive care treatment, need for specific treatment of Covid disease, antibiotic treatment) Subjective presence of residual symptoms of Covid disease OR no residual symptoms of Covid disease (Controls) Informed consent  Exclusion Criteria:  Continuous probiotic treatment in the last 4 weeks before inclusion Pre-existing lung diseases