By now, we all should have memorized the key symptoms of the novel coronavirus that public health officials have hammered home: fever, cough, and trouble breathing.

However, that message may have given some who thought they had to have all three symptoms at once a false sense of security.

The earliest reports on the disease caused by COVID-19 were based on patients sick enough to see a doctor or go to a hospital. Many had the three symptoms.

As we practice social distancing, though, an important question becomes: How does this disease start? Knowing the early symptoms could help people know when to isolate themselves even more and talk to their doctors about testing.

While hundreds of thousands of people have contracted COVID-19 over the last few months, information about its earliest stages has been slow to trickle out. Doctors said there are still important gaps in what they know. And, unfortunately, many of the early symptoms of COVID-19 overlap with other common health problems.

“There is no clearly defined syndrome at this point," said Nuala Meyer, a pulmonary and critical-care medicine doctor at Penn Medicine.

What we know

Symptoms can be subtle. It is known that some people infected have no symptoms or ones so mild that they’re not noticed.

Daniel Mueller, an infectious diseases doctor at Temple University Hospital, said infected people might feel “just slightly under the weather for a few days.… They might just feel a little tired or a little achy." It’s the sort of feeling you’d have if you thought you were coming down with something, but nothing very worrisome.

Then they might start having a dry cough. Later, in some people, shortness of breath comes on. “They don’t always happen together,” Mueller said.

It’s not like the flu. Unlike the flu, which announces itself suddenly with fever and muscle aches, the early stages of COVID-19 can go on for a few days.

Reynold A. Panettieri Jr., vice chancellor for clinical and translational science at Rutgers University, said some mild cases last just three to four days. Others who are not sick enough for hospitalization still may have symptoms for two weeks. Presentation of the disease is, he said, “very individualistic and very heterogeneous." Some people are barely affected and some have “shaking chills” and muscle aches, he said. Shortness of breath may manifest in two to three days. More severe symptoms may portend a rougher course, but it’s not yet clear if they do.

“You just don’t know whether they’re going to progress," he said. In severe cases, patients develop pneumonia and need help breathing. “The susceptibility to progression is unclear.”

Fever? Yes and no. Meyer would add sore throat to the list of early symptoms. She said most patients will eventually have a fever, but it “may not be in the first five days.”

A report about the outbreak in China published in the New England Journal of Medicine said about 44% of COVID-19 patients had a fever when admitted to a hospital and 88% developed one during their stay. The researchers warned that cases could be missed if doctors considered fever part of the case definition.

Gastrointestinal symptoms and spread

A rarely mentioned early symptom is gastrointestinal distress. About 10% of cases can start with a couple of days of diarrhea, abdominal pain, nausea, or vomiting and then respiratory symptoms develop, said David A. Johnson, chief of gastroenterology at Eastern Virginia Medical School, who has reviewed studies on the new disease. He said this makes sense because the virus first targets a type of cell that lives both in the lungs and in parts of the digestive tract.

There is evidence of viral shedding in feces, he said. That is not proof that virus in the stool is infectious, he said, but is reason to worry that the disease can be transmitted through a fecal-oral route. It’s another argument for good hand hygiene and bathroom cleaning. In a Chinese study, the shedding continued for up to 12 days after people tested negative for the virus, he said.

Gavin Macgregor-Skinner, an assistant professor in the department of public health at the Penn State University College of Medicine, trains people in hospitals and other organizations in infection prevention and control. He said he is frustrated by the lack of information this many weeks into a pandemic. He said medical journals can be too slow to provide information, and doctors should find quicker ways, such as a clearinghouse, to share data.

For now, he is asking the 37 members of his prevention team to take their own temperatures in the morning and evening. That way they’ll know their personal baseline. If the temperature goes up, he wants them to stop working and test every three hours.

He said he doesn’t know whether fever is the first symptom, but added that low-grade fever is “definitely present” in the disease. He also wants people to stay away from others if they have any respiratory symptoms. He agreed that breathing difficulty is not an early sign of the disease.

He didn’t know about gastrointestinal symptoms, but he had strong words about infection control when fecal-oral transmission is a possibility. Active virus in feces can spray into the room when you flush; so, close the lid when flushing. You shouldn’t have personal items near the toilet, he said: “Don’t have your toothbrush next to your toilet, ever.”

Mueller said the three major symptoms — fever, cough, and shortness of breath — warrant a call to the doctor and a discussion about whether to be tested. He would set the threshold lower for transplant patients or those with chronic heart and lung disease. Shortness of breath could be a sign of pneumonia and should get immediate attention, he said.

Symptoms involving the eyes and nose

The American Academy of Ophthalmology on March 25 issued a warning to its members that some patients with conjunctivitis, an inflammation of the thin, transparent tissue that lines eyelids and part of the eye also known as pink eye, can be an early sign of coronavirus. It suggested that eye doctors wear full face protection covering eyes, ears and mouth, particularly when patients also have fever and respiratory symptoms.

The organization cited two studies from China that involved patients hospitalized for the new disease. One found viral conjunctivitis in 1 of 30 patients. The other found it in 9 of 1,099 (0.8%) lab-confirmed cases.

British ear, nose and throat doctors have also raised the alarm that loss of the sense of smell can be a symptom.

In a statement, ENTUK said there is “good evidence” from South Korea, China and Italy that pepole with COVID-19 infection can experience total loss of the sense of smell (asnosmia) or a decline. “In Germany it is reported that more than two in three confirmed cases have anosmia,” they wrote. “In South Korea, where testing has been more widespread, 30% of patients testing positive have had anosmia as their major presenting symptom in otherwise mild cases.”