"The act of smelling something, anything, is remarkably like the act of thinking," said Lewis Thomas, physician, poet, and author of "Lives of the Cell." Smells evoke layers of sensations, stimulate memory, and lead to pleasure or repugnance.

Without a sense of smell, everything from food and flowers to air, water, and even other people becomes far less engaging.

For some people, it also causes distortions in odor and taste; what was previously pleasant becomes distasteful or repulsive.

Up to 1.6 million people may be having chronic difficulty detecting smells because of COVID-19, according to a research letter published in JAMA Otolaryngology — Head & Neck Surgery. That's in addition to the 13.3 million U.S. adults ages 40 and older who have a non-COVID-19-related loss of ability to smell (olfactory dysfunction).

There are many potential causes of olfactory dysfunction, including head trauma, diabetes, some cancers, zinc deficiency, nasal polyps, viral infections, and allergies. If the loss of smell persists or isn't treatable with medications (for allergies) or surgery (to remove polyps), it can trigger depression (in 40% of patients), inappropriate weight loss (food has no taste), and nutritional deficiencies.

Experts recommend that after two weeks of loss of smell, people undergo olfactory training for at least three months. This involves sniffing a set of odors — typically lemon, rose, cloves, eucalyptus, and coffee — for 20 seconds each (with eyes open) at least twice a day.

This will help set up new neural pathways and retrain your brain.