Until March, when everything started tasting like cardboard, Katherine Hansen had such a keen sense of smell that she could recreate almost any restaurant dish at home without the recipe, just by recalling the scents and flavors.

Then the coronavirus arrived. One of Ms. Hansen’s first symptoms was a loss of smell, and then of taste. Ms. Hansen still cannot taste food, and says she can’t even tolerate chewing it. Now she lives mostly on soups and shakes.

“I’m like someone who loses their eyesight as an adult,” said Ms. Hansen, a realtor who lives outside Seattle. “They know what something should look like. I know what it should taste like, but I can’t get there.”

A diminished sense of smell, called anosmia, has emerged as one of the telltale symptoms of Covid-19, the illness caused by the coronavirus. It is the first symptom for some patients, and sometimes the only one. Often accompanied by an inability to taste, anosmia occurs abruptly and dramatically in these patients, almost as if a switch had been flipped.

Most regain their senses of smell and taste after they recover, usually within weeks. But in a minority of patients like Ms. Hansen, the loss persists, and doctors cannot say when or if the senses will return.

Scientists know little about how the virus causes persistent anosmia or how to cure it. But cases are piling up as the coronavirus sweeps across the world, and some experts fear that the pandemic may leave huge numbers of people with a permanent loss of smell and taste. The prospect has set off an urgent scramble among researchers to learn more about why patients are losing these essential senses, and how to help them.

“Many people have been doing olfactory research for decades and getting little attention,” said Dr. Dolores Malaspina, professor of psychiatry, neuroscience, genetics and genomics at Icahn School of Medicine at Mount Sinai in New York. “Covid is just turning that field upside down.”

Smell is intimately tied to both taste and appetite, and anosmia often robs people of the pleasure of eating. But the sudden absence also may have a profound impact on mood and quality of life.

Studies have linked anosmia to social isolation and anhedonia, an inability to feel pleasure, as well as a strange sense of detachment and isolation. Memories and emotions are intricately tied to smell, and the olfactory system plays an important though largely unrecognized role in emotional well-being, said Dr. Sandeep Robert Datta, an associate professor of neurobiology at Harvard Medical School.

“You think of it as an aesthetic bonus sense,” Dr. Datta said. “But when someone is denied their sense of smell, it changes the way they perceive the environment and their place in the environment. People’s sense of well-being declines. It can be really jarring and disconcerting.”

Many sufferers describe the loss as extremely upsetting, even debilitating, all the more so because it is invisible to others.

“Smell is not something we pay a lot of attention to until it’s gone,” said Pamela Dalton, who studies smell’s link to cognition and emotion at the Monell Chemical Senses Center in Philadelphia. “Then people notice it, and it is pretty distressing. Nothing is quite the same.”

British scientists studied the experiences of 9,000 Covid-19 patients who joined a Facebook support group set up by the charity group AbScent between March 24 and September 30. Many members said they had not only lost pleasure in eating, but also in socializing. The loss had weakened their bonds with other people, affecting intimate relationships and leaving them feeling isolated, even detached from reality.

“I feel alien from myself,” one participant wrote. “It’s also kind of a loneliness in the world. Like a part of me is missing, as I can no longer smell and experience the emotions of everyday basic living.”

Another said, “I feel discombobulated — like I don’t exist. I can’t smell my house and feel at home. I can’t smell fresh air or grass when I go out. I can’t smell the rain.”

Loss of smell is a risk factor for anxiety and depression, so the implications of widespread anosmia deeply trouble mental health experts. Dr. Malaspina and other researchers have found that olfactory dysfunction often precedes social deficits in schizophrenia, and social withdrawal even in healthy individuals.

“From a public health perspective, this is really important,” Dr. Datta said. “If you think worldwide about the number of people with Covid, even if only 10 percent have a more prolonged smell loss, we’re talking about potentially millions of people.”

The most immediate effects may be nutritional. People with anosmia may continue to perceive basic tastes — salty, sour, sweet, bitter and umami. But taste buds are relatively crude preceptors. Smell adds complexity to the perception of flavor via hundreds of odor receptors signaling the brain.

Many people who can’t smell will lose their appetites, putting them at risk of nutritional deficits and unintended weight loss. Kara VanGuilder, who lives in Brookline, Mass., said she has lost 20 pounds since March, when her sense of smell vanished.

“I call it the Covid diet,” said Ms. VanGuilder, 26, who works in medical administration. “There no point in indulging in brownies if I can’t really taste the brownie.”

But while she jokes about it, she added, the loss has been distressing: “For a few months, every day almost, I would cry at the end of the day.”

Smells also serve as a primal alarm system alerting humans to dangers in our environment, like fires or gas leaks. A diminished sense of smell in old age is one reason older individuals are more prone to accidents, like fires caused by leaving burning food on the stove.

Michele Miller, of Bayside, N.Y., was infected with the coronavirus in March and hasn’t smelled anything since then. Recently, her husband and daughter rushed her out of their house, saying the kitchen was filling with gas.

She had no idea. “It’s one thing not to smell and taste, but this is survival,” Ms. Miller said.

Humans constantly scan their environments for smells that signal changes and potential harms, though the process is not always conscious, said Dr. Dalton, of the Monell Chemical Senses Center.

Smell alerts the brain to the mundane, like dirty clothes, and the risky, like spoiled food. Without this form of detection, “people get anxious about things,” Dr. Dalton said.

Even worse, some Covid-19 survivors are tormented by phantom odors that are unpleasant and often noxious, like the smells of burning plastic, ammonia or feces, a distortion called parosmia.

Eric Reynolds, a 51-year-old probation officer in Santa Maria, Calif., lost his sense of smell when he contracted Covid-19 in April. Now, he said, he often perceives foul odors that he knows don’t exist. Diet drinks taste like dirt; soap and laundry detergent smell like stagnant water or ammonia.

“I can’t do dishes, it makes me gag,” Mr. Reynolds said. He’s also haunted by phantom smells of corn chips and a scent he calls “old lady perfume smell.”

It’s not unusual for patients like him to develop food aversions related to their distorted perceptions, said Dr. Evan R. Reiter, medical director of the smell and taste center at Virginia Commonwealth University, who has been tracking the recovery of some 2,000 Covid-19 patients who lost their sense of smell.

One of his patients is recovering, but “now that it’s coming back, she’s saying that everything or virtually everything that she eats will give her a gasoline taste or smell,” Dr. Reiter said.

The derangement of smell may be part of the recovery process, as receptors in the nose struggle to reawaken, sending signals to the brain that misfire or are misread, Dr. Reiter said.

After loss of smell, “different populations or subtypes of receptors may be impacted to different degrees, so the signals your brain is used to getting when you eat steak will be distorted and may trick your brain into thinking you’re eating dog poop or something else that’s not palatable.”

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Patients desperate for answers and treatment have tried therapies like smell training: sniffing essential oils or sachets with a variety of odors — such as lavender, eucalyptus, cinnamon and chocolate — several times a day in an effort to coax back the sense of smell. A recent study of 153 patients in Germany found the training could be moderately helpful in those who had lower olfactory functioning and in those with parosmia.

Dr. Alfred Iloreta, an otolaryngologist at Mount Sinai Hospital in New York, has begun a clinical trial to see whether taking fish oil helps restore the sense of smell. The omega-3 fatty acids found in fish oil may protect nerve cells from further damage or help regenerate nerve growth, he suggested.

“If you have no smell or taste, you have a hard time eating anything, and that’s a massive quality of life issue,” Dr. Iloreta said. “My patients, and the people I know who have lost their smell, are completely wrecked by it.”

Mr. Reynolds feels the loss most acutely when he goes to the beach near his home to walk. He no longer smells the ocean or salt air.

“My mind knows what it smells like,” he said. “And when I get there, it’s not there.”