During the COVID-19 pandemic, it became clear that smell and taste (chemosensory) disturbance is very common in the early stages of disease. This article addresses (i) why COVID-19 specifically targets the modalities of smell and possibly taste and what is the mechanism, (ii) what is the frequency of smell and taste loss and (iii) what is the overall prognosis. It is suggested that mouth-breathers may be at particular risk of COVID-19. Symptom-based questionnaires are likely to under-estimate the prevalence of chemosensory impairment by as much as 50%. The prevalence of smell loss is so high that a person who has normal olfaction on formal testing is unlikely to be infected significantly with Cov-2. Furthermore, someone without symptoms who has an abnormal smell test could still be infected and liable to spread the disease. Brief, low-cost, olfactory tests are available that would permit a high throughput in field stations and airports. A normal result might obviate the need for a nasopharyngeal swab for the Cov-2 virus.