Methods: all studies are case control studies on six different groups: normal individuals, non-COVID-19 patients with olfactory dysfunction, COVID-19 positive patients, previous COVID-19 positive patients with prolonged olfactory dysfunction, patients with chronic rhinosinusitis with and without nasal polyps, and patients with CRS initiating intranasal glucocorticoids.. The patients will be included from the ear-, nose, and throat department and the University Clinic for Flavour, Balance, and Sleep Regional Hospital Gødstrup, as well as Flavour Institute, Department of Clinical Medicine, Aarhus University (AU), and the long COVID-19 clinics in Aarhus and Gødstrup. Microbiome samples will be swabs from the nasal cavity and oral pharynx will be collected and analysed using next-generation sequencing targeting 16S and 18S ribosomal RNA at the Department of Bacteria, Parasite \& Fungi at Staten Serum Institute (SSI), Copenhagen, Denmark. Characterization of the microbiome is performed by 16S/18S amplicon-based metagenomics, and data are analyzed by various bio-informatic tools for determination/visualization of alpha/beta diversity as well as principal coordinates analysis (PCoA). Immunological factors will be collected by means of six swabs: in the nasal cavity and used for either flowcytometry or enzyme-linked immunosorbent assay (ELISA). For comparison, truecut biopsies of the nasal mucosa are collected from a subgroup of patients undergoing surgery in general anaesthesia. Due to the novelty of the current study, little relevant information from previous literature can be used for power calculation. However, with an alpha of 5% and a power of 80%, the population size for each group should be 42 participants, assuming the occurrence of bacteria differs with 30% between populations. Taking 20% drop out into account, the investigators aim to include 60 patients in each group. Perspectives: The perspectives of this study is to begin bridging the gap between gut and upper airway microbiome and immunology research. Looking at the explosion of interest in the gut, and in personalized medicine in general, the investigators likewise foresee a huge potential for both general and individual prevention and treatments starting with a deeper understanding of the nasal microbiome and immunology. Examples beyond the scope of this study could be microbiome transplants for chronic sufferers of diseases such as chronic rhinosinusitis, allergies or asthma, or in selecting which patients could reap the benefits of the expensive biological treatments emerging on the market.