For a Texas nurse, the first sign that something was wrong happened while brushing her teeth — she couldn’t taste her toothpaste. For a Georgia attorney, it was hitting a wall of fatigue on a normally easy run. When a Wisconsin professor fell ill in June, he thought a bad meal had upset his stomach.

But eventually, all of these people discovered that their manifold symptoms were all signs of Covid-19. Some of the common symptoms — a dry cough, a headache — can start so mildly they are at first mistaken for allergies or a cold. In other cases, the symptoms are so unusual — strange leg pain, a rash or dizziness — that patients and even their doctors don’t think Covid-19 could be the culprit.

With more than 18 million cases of coronavirus worldwide, one thing is clear: The symptoms are varied and strange, they can be mild or debilitating, and the disease can progress, from head to toe, in unpredictable ways.

Despite hundreds of published studies on Covid-19 symptoms, just how common any given symptom is depends on the patient group studied. Patients in hospitals typically have more severe symptoms. Older patients are more likely to have cognitive problems. Younger patients are more likely to have mild disease and odd rashes.

“The problem is that it depends on who you are and how healthy you are,” said Dr. Mark A. Perazella, a kidney specialist and professor of medicine at Yale School of Medicine. “It’s so heterogeneous, it’s hard to say. If you’re healthy, most likely you’ll get fever, achiness, nasal symptoms, dry cough and you’ll feel crappy. But there are going to be the oddballs that are challenging and come in with some symptoms and nothing else, and you don’t suspect Covid.”

The Texas nurse who couldn’t taste her toothpaste said she developed fever, “horrible” body aches and coughing the next day. Her symptoms lasted for five days. (She and many others interviewed asked that their names not be used to protect their medical privacy or to protect their families from the stigma of Covid-19.)

Anosmia, the loss of sense of smell that is also often accompanied by a loss of taste, is viewed as a defining symptom. In a study of 961 health care workers who were tested for Covid-19, anosmia was the most predictive symptom, but it wasn’t foolproof. Only half the people who reported losing their sense of smell or taste tested positive, said Dr. Brian Clemency, the study’s lead author and an associate professor in the department of emergency medicine at the Jacobs School of Medicine and Biomedical Sciences at the University at Buffalo.

Even a symptom as common as fever can be tricky when trying to predict if a patient might have Covid-19. Although many businesses are doing fever checks to screen for Covid-19, many Covid-19 patients never have a fever. In a European study of 2,000 Covid-19 patients with mild to moderate illness, 60 percent never had a fever. In the University at Buffalo study, fewer than one in three patients with fever also tested positive for Covid-19.

Rob Gregson, 52, of South Orange, N.J., went to bed feeling under the weather and woke up with chest tightness, a “weird” cough, difficulty breathing and “crazy fatigue.” It was March 11, just before lockdowns were imposed, and he immediately suspected Covid-19. But because he never had a fever, it took him more than a week to find a doctor to help and get a swab test. He tested positive.

“It’s been the fatigue that is the most debilitating,” said Mr. Gregson, executive director of a faith-based nonprofit, adding that he’s still struggling to regain his stamina nearly five months later. “I’ve been on the coronavirus roller coaster, feeling better and thinking I’ll be OK, then it comes roaring back.”

When Erin, a 30-year-old who works for a nonprofit in Washington, D.C., first developed a cough and headache in May, she wasn’t worried. “I did not have a fever, and I’d been very diligent about wearing a mask and washing my hands, so I figured it was allergies or a cold at the beginning,” she said.

About four days after the cough began, Erin was hit with severe fatigue, sore throat, congestion, chills, body aches and a slight loss of sense of smell — but still no fever. She also had one unusual symptom: severe pain in her hip muscles, which she described as “really weird.”

Although body aches are a common symptom of Covid-19, some patients are reporting severe joint and body pain, particularly in large muscles. Although it’s rare, Covid-19 can cause painful inflammation in the joints or lead to rhabdomyolysis, a serious and potentially life-threatening illness that can cause excruciating muscle pain in the shoulders, thighs or lower back.

A New York cyclist who developed severe leg pain in May was initially diagnosed via telemedicine with a bulging disc. She sought a second telemedicine opinion with Dr. Jordan Metzl, a sports medicine specialist at the Hospital for Special Surgery in New York, who asked her to move, twist and put pressure on her legs as he watched her on video.

“Down to her calf she said, ‘Ouch, that really hurts,’” said Dr. Metzl, who grew worried. “I’m not an alarmist doctor in the least, but I looked for the closest emergency room to her, which was 16 miles away. I said, ‘I want you to get in the car and drive yourself to the E.R. right now.’”

An ultrasound showed she had no pulse in her legs and severe clotting in both legs, putting her at risk of amputation. She was transferred to another hospital and underwent nine hours of emergency surgery. Dr. Metzl said it was fortunate that he had just had a conference call with colleagues about blood clots and Covid.

“It’s a terrifying story, which is why we need awareness around these weird presentations,” Dr. Metzl said. “Covid infection can affect different body parts differently. Some people get this hypercoagulable state and end up getting blood clots. We don’t always know who those people are.”

In June, John, a 55-year-old professor in Oshkosh, Wis., woke up one morning feeling like something he had eaten disagreed with him. The next day he was hit with debilitating fatigue and nausea, cramping and other gastrointestinal symptoms. He didn’t suspect Covid-19 because he had been wearing a mask and social distancing.

“By the second day I was just wanting to sleep all the time. I was probably sleeping 20 hours a day,” he said. “I even remember during that time my mind set changed. I could not imagine how my wife and son were able to be awake all day. I thought, ‘How can anyone possibly do that?’ I would get out of bed and go to the kitchen for a glass of water, and by the time I got there all I could think about was wanting to go back to bed.”

Doctors tested him for Covid-19 and Lyme disease. Both were negative. An ultrasound showed no problems, but blood work suggested he had an infection of some kind. A doctor prescribed a two-week course of a heartburn drug, and he lost 10 pounds. After two weeks, he began feeling better. Two months later, he tested positive for Covid-19 antibodies, suggesting his original Covid test had been a false negative.

Doctors say Covid patients with only gastrointestinal symptoms often test negative when tested with a nasal-pharyngeal swab. The virus might be more likely to show up in fecal testing, which is common in other countries, but not widely used in the United States. The gastrointestinal tract and the respiratory tract both are rich in a receptor called ACE2, which the virus uses to get into our cells. But it’s unclear why the virus sometimes seems to skip the respiratory tract and instead infects only the digestive tract.

“This is a very tricky and confounding virus and disease, and we are finding out surprising things about it every day,” said Dr. Asaf Bitton, executive director of Ariadne Labs at Brigham and Women’s Hospital and the Harvard T.H. Chan School of Public Health.

Dr. Ilan Schwartz, assistant professor of infectious diseases at the University of Alberta, said he was tested for Covid-19 after developing respiratory symptoms. The test came back negative, but then he developed Covid toes — painful red or purple lesions on the tips of fingers and toes that are believed to be a telltale symptom of coronavirus, particularly in younger patients. It may happen as a result of small blood clots or when the virus invades blood vessels.

“I started getting these sores on my feet and couldn’t figure out what was going on,” said Dr. Schwartz, who is 37. “They were really painful. I thought maybe I had stubbed them — like all of them, which would be unusual. Then I thought maybe there’s something wrong with my shoes. I’ve heard a lot of people with similar stories end up buying new shoes because they don’t know what’s going on. It’s such an unusual symptom that it’s not natural to think of a respiratory virus being responsible for sore toes.”

Thomas Ryan, 36, an Atlanta attorney, said the first sign that something was wrong hit him during exercise.

“I went for a run on a Thursday afternoon after work and felt awful,” he said. “I hit the wall like you do in a marathon on a very short run for me.”

The next morning, he woke up with a light cough, sore throat and a feeling in his chest like heartburn, and later developed fatigue, lung pain and shortness of breath. Although his Covid test was negative, his doctor told him that it was a false negative, and that based on his symptoms, he clearly had Covid-19.

“This is not great,” said Mr. Ryan, who was still coughing weeks after falling ill. “The amount of energy I have — I feel like I’m at altitude. It was two weeks of not being able to do anything. If this is a mild case, it makes me think people are taking a lot of risks they probably shouldn’t be.”