The prevailing wisdom is (that post-Covid) parosmia is a good thing – a sign of the neurons regenerating but failing to make their connection, like leads in the wrong port. But, [Barry Smith of the Global Consortium of Chemosensory Research] tells me, it’s not a simple “miswiring”. There appear to be certain volatile molecules, such as those found in coffee and roasted meat – which in high concentration would be quite toxic – that set it off, perhaps triggering a kind of warning alarm.

“It’s a bit like tinnitus in the ears,” Smith suggests, in that the brain is struggling to modulate the signals it receives; my nose is whining, always at the same high pitch.

— Read more in Searching for sense: what we lose when smell deserts us at New Statesman.

[Charles Wysocki of Monell Chemical Senses Center] found 20 volunteers who also could not initially detect androstenone, and after six weeks of sniffing the pheromone for three minutes, three times a day, half of them became sensitive to the molecule. This finding is in some ways intuitive; the olfactory senses of perfumers and sommeliers, after all, become more attuned to perfumes and wine with time.

— Read more about the development of post-Covid-19 smell training in You Recovered From COVID-19. Now Your Coffee Smells Like Sewage. at The Atlantic.

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.