Barrie Smith was diagnosed with Parkinson's Disease in his 50s, but 18 years before he was given his diagnosis, he developed a very strange and permanent symptom. One day he detected "a strong smoky smell, like burning wires", he says. Since then, he has never smelt anything again. At the time, flummoxed, Smith's doctor attributed the loss of his sense of smell to his scuba diving, as deep diving is known to sometimes cause smell impairments. If only the doctor was right.

Smell loss can have sinister origins beyond scuba diving, and in Smith's case, that proved to be true. In today's world, most would automatically attribute the loss of smell to Covid-19, but it is also a common symptom of neurodegenerative diseases, including Multiple Sclerosis, Parkinson's disease and Alzheimer's disease.

— Read more about efforts to develop smell-related diagnostic tests (and how to retrain your olfactory system) in How your sense of smell predicts your overall health at BBC.