Close-up of a person being infected by SARS-CoV-2 particles (blue), the cause of Covid-19 and the olfactory system, the sensory system responsible for smell. The olfactory bulb (pink, upper centre) is located in the forebrain. It has many olfactory receptor cells that project into the lining of the nasal cavity and detect smell molecules. The olfactory bulb relays information from the receptors to other parts of the brain. Loss of smell (anosmia) is a common symptom of Covid-19, especially in those that are otherwise asymptomatic. The exact mechanism by which the virus causes anosmia is not known.