Scientists and public-health officials are learning more about the new coronavirus behind a continuing pandemic. The disease it causes is called Covid-19. We are updating our questions and answers regularly to keep up with their findings. Here is what they know so far, and how you can minimize your risk.

What are the symptoms of Covid-19?

The virus infects both the upper and lower respiratory tract. That means it is contagious, and can cause pneumonia. People who are infected initially develop a fever, cough and aches, and can progress to shortness of breath and complications from pneumonia, according to case reports. Of nearly 56,000 patients in China, 87.9% had a fever, 67.7% had a dry cough, 38.1% experienced fatigue and 18.6% had shortness of breath. Other, less common symptoms included sore throat, headache, aches, chills, repeated shaking with chills, vomiting, diarrhea and nasal congestion. Some people report losing the senses of taste and smell.

THE LATEST ON THE CORONAVIRUS Johns Hopkins: Coronavirus-related deaths in the U.S. pass 97,800; infections more than 1.6 million

More than 5.4 million people have been infected globally, with over 345,000 deaths

Some people become only mildly ill, or are infected but don’t get sick. Others are mildly ill for a few days, then rapidly develop more severe symptoms of pneumonia.

The U.S. Centers for Disease Control and Prevention says that people with severe symptoms should seek medical treatment immediately. Those severe symptoms include:

• Difficulty in breathing or shortness of breath

• Persistent pain or pressure in the chest

• Bluish lips or face

What should I do if I develop symptoms?

If you think you have been exposed and develop symptoms, call your doctor. Ask if you can be tested. He or she will likely test you first for other respiratory infections such as the flu.

Isolate yourself from others and limit contact with pets as well. Cover your mouth and nose with a tissue when you cough and sneeze. Follow the same precautions you would to prevent infection, washing hands regularly. Wear a face mask, if you have one, when around others at home.

Epidemic Potential

Coronaviruses are jumping increasingly from animals to humans, creating new threats

NL63 (1100s) SARS (2003) 229E (1800s) MERS (2012) OC43 (1800s) SARS-CoV-2 (2020) HKU1 (unknown)

How worried should I be?

Most people who are infected might become only mildly ill, data suggest. But “mild” can be anything from a fever, cough and aches to pneumonia. So for most people it is probably not just a few sniffles. And mild or not, you’ll have to be isolated or quarantined.

By not getting infected, you would also protect those around you, including older family members or anyone you know with heart disease or diabetes, conditions that increase risk of severe illness.

Of 44,672 cases in China, 81% had mild symptoms, 13.8% were severely ill, and 4.7% were critically ill, according to the Chinese CDC. All of those who died were in critical condition.

Public-health officials are trying to determine how many people have been infected, including those who didn’t get sick at all. They are concerned and want to contain the virus because its effects aren’t fully known. In addition, new viruses can mutate, possibly becoming more virulent as they work their way through a population.

Related Video Public-health officials hope to ‘flatten the curve’ of infections with social-distancing measures.

I’m social distancing and I feel fine. Am I in the clear?

The new coronavirus is spreading in many cities, and as long as it is spreading at this rate, there is a risk of getting it. You may not show symptoms right away. People become ill between two and 14 days after infection, or in an average of about 5 days, according to most estimates.

Is taking a common, over-the-counter cold medication helpful?

Experts say this is helpful for controlling symptoms, which is the mainstay of treating the new coronavirus. But it isn’t a cure and won’t prevent you from infecting others.

I’ve heard I shouldn’t take ibuprofen. Is that true?

There have been reports that use of painkillers from a class known as nonsteroidal anti-inflammatory drugs, containing ingredients like ibuprofen, might worsen Covid-19 infections. Ibuprofen is known to reduce inflammation in the body that may be needed to fight an infection. But the World Health Organization says it doesn’t recommend against the use of ibuprofen. The public health agency said it isn’t aware of evidence on the topic or negative effects in patients from ibuprofen, beyond the usual side effects. It says it is consulting with physicians who are treating patients.

Is there a test?

Yes, there are diagnostic tests that you can get through a doctor or hospital, which are the only way to confirm for certain whether a patient has the new coronavirus or another infection. Hundreds of thousands of people have been tested in China and South Korea. In the U.S., testing was limited by problems with a CDC-developed test and narrow testing criteria. Now, more tests are being distributed and the CDC says doctors may decide whether a patient should be tested.

A physician assistant demonstrated use of a protective booth for administering Covid-19 tests at a Brigham and Women’s Hospital community site in Boston. Photo: brian snyder/Reuters

Blood tests are being developed and licensed to test people for antibodies to the virus, to determine how many have been infected. Some may not have gotten sick. That test could also show whether people who did get sick are immune from reinfection.

What if I have to self-isolate?

If you are told to self-isolate, you will need to stay at home and avoid contact with others for 14 days. Try not to stay in the same room with others at the same time, the U.K.’s National Health Service recommends. Stay in a well-ventilated room with a window that can be opened. Don’t share towels, utensils or dishes with others, and wash them thoroughly after use. Clean bathrooms and surfaces regularly. Wash your hands before and after contact with pets.

Don’t go out to public places; ask family members or friends to get groceries, medicines and other supplies for you. Ask delivery people to leave items outside.

Are there drugs to treat coronaviruses?

There aren’t any drugs or vaccines approved specifically for the new virus. But more than three dozen are in development or being studied. The first human testing of Moderna Inc.’s experimental vaccine against the virus has begun at the Kaiser Permanente Washington Health Research Institute in Seattle. While this first part is starting early, the estimate still is that testing of the vaccine will take a year to 18 months to complete. A few other vaccine makers are developing products targeting the virus.

Two clinical trials in China and one in the U.S. are evaluating remdesivir, an antiviral drug from Gilead Sciences Inc. that was also tested for Ebola. A malaria drug, hydroxychloroquine, has gained attention as a possible treatment, but there is no scientific evidence that it works, experts say.

How do I keep from getting Covid-19?

Schools, workplaces and other public gathering places have been closed in many places, and officials are recommending—and in some cases, requiring—that people remain in their homes and away from exposure to other people while the coronavirus continues to spread.

confirmed cases in the U.S. total deaths in the U.S. Note: Last updated .

Source: Johns Hopkins Center for Systems Science and Engineering

Who is most at risk?

Adults of all ages have been infected, but the risk of severe disease and death is highest for older people and those with other health conditions such as heart disease, chronic lung disease, cancer and diabetes. Most of the 1,023 people whose deaths were included in a study by the Chinese Center for Disease Control and Prevention were age 60 or older, and/or had other illnesses. Many patients who have died were admitted to hospitals when their illness was advanced. One large study in China found a mortality rate of 14.8% in people ages 80 and older, and 8% in people ages 70 to 79, compared with a 2.3% mortality rate overall in the population studied.

Wendy Greene, a physician at Emory Hospital, volunteered at a pop-up test site in Decatur, Ga., administering Covid-19 tests. Photo: Curtis Compton/Atlanta Journal-Constitution/Associated Press

Are my children at risk?

Few children have been reported with the infection, but that could change. Of the children who were infected in China, only a small proportion were severely ill, according to the WHO. While the disease is mild for teens and younger adults, some have had severe illnesses and died. Of 121 U.S. patients admitted to an intensive care unit, 12% were adults between the ages of 20 and 44, according to a study by the U.S. CDC.

How at risk is someone in the U.S.?

Covid-19 has spread rapidly, with cases identified in all 50 states and large outbreaks in some areas. Federal, state and local officials are taking unprecedented steps to try to minimize transmission, such as banning large gatherings and imposing restrictions on movement. Everyone in the U.S. is being advised to adhere to these measures for their own safety and to protect others, particularly those at highest risk.

The number of confirmed cases is substantially lower than the number of actual infections, public-health officials say, because of a shortage of tests and because many people who get infected might not be sick enough to realize it.

Public-health officials hope to “flatten the curve” of infections with the social-distancing measures, slowing the spread of the virus so that fewer people are infected. That’s important to keep more people healthy and prevent hospitals from becoming too overwhelmed to care for all of the sick.

How is the virus spread among humans?

It transmits through “respiratory droplets” when an infected person speaks, coughs or sneezes, according to the WHO. The droplets spread through the air and can land on another person’s mouth or nose, or possibly be inhaled into their lungs, infecting them. The droplets can also settle on nearby surfaces like a desk, counter or doorknob, where they can survive for a period. A person can become infected by touching a contaminated surface, then touching their mouth, nose or eyes.

Respiratory droplets are heavy and don’t travel far in the air, so transmission is believed to occur mostly through close contact, meaning within 6 feet of an infected person.

Scientists are also investigating whether the new coronavirus might spread through urine or feces. Tests have found it in the digestive tract of some patients.

How long can the virus survive on surfaces?

It can last up to 72 hours on plastic and stainless steel, up to 24 hours on cardboard and four hours on copper, according to a study in the New England Journal of Medicine by U.S. government and academic scientists. But it degrades quickly, said Vincent Munster, a virologist with the National Institute of Allergy and Infectious Diseases who led the research.

The study also showed that the virus can survive in small droplets, known as aerosols, for about three hours, though it disintegrates over time. These droplets are small enough to remain suspended in the air for half an hour to an hour, depending on air flow, Dr. Munster said.

Wipe down countertops, doorknobs and other commonly touched surfaces frequently. Regular household disinfectant wipes and cleaners kill the virus.

Can face masks protect you?

Health experts and mask makers say only a properly used reusable N95 respirator mask certified by an independent agency can guard fully against the virus. Surgical masks and even cloth masks can provide some protection, though. The U.S. CDC recommends that people wear cloth masks or face coverings—even a bandanna or scarf will do—in public places like grocery stores where social distancing may be difficult.

A worker, left, in the Louis Vuitton store at the International Mall in Tampa, Fla., assisted customers on May 6. Photo: Chris O'Meara/Associated Press

Cloth masks or coverings don’t fully protect wearers from becoming infected because they aren’t completely sealed on the face, according to experts. But they can help prevent people who are infected from spreading the virus by catching droplets emitted while exhaling, coughing or sneezing. They shouldn’t be used on children under 2 years old or anyone who has trouble breathing, the CDC says. The CDC asks people to use nonmedical masks because supplies of masks are short and should be saved for health-care workers and caregivers.

What else can I do to protect myself?

The most important thing you can do is wash your hands frequently, for at least 20 seconds each time. Wash them regularly when you are at the office, when you come home, before you eat and other times that you are touching surfaces. You can also use a hand sanitizer that contains at least 60% alcohol. Don’t touch your eyes, nose or mouth—viruses can enter your body that way. Wipe down objects and surfaces frequently with household cleaner, which will kill the virus. Maintain a distance from people who are sick. Stay about 6 feet or more away from others.

An electronic sign in the Staten Island Ferry’s Whitehall Terminal in New York City promoted social distancing on Wednesday. Photo: Krista Kennell/Zuma Press

Stock up on some supplies when recommended to stay home, infectious-disease experts say. Items to consider include shelf-stable foods like cans of beans, packages of rice and pasta, and beverages; pain relievers and other common medications; extra prescription medications; and hygiene and cleaning products.

Is it safe to travel internationally?

The State Department advises U.S. citizens to avoid all travel abroad due to the global impact of Covid-19, and has urged Americans currently abroad to return home immediately. Citizens living abroad are also advised to avoid all international travel. Those who are in countries where outbreaks are occurring should stay home as much as possible, limit contact with others and follow guidelines from the CDC to prevent infection, U.S. authorities say.

Can I donate blood?

The CDC, American Red Cross and U.S. Surgeon General are encouraging people to donate blood, if they are able and healthy. “Social distancing does not have to mean social disengagement,” said Surgeon General Jerome Adams.

To minimize risks of exposure to the new coronavirus, the CDC has recommended that blood centers keep donor chairs 6 feet apart and encouraged people to make appointments ahead of time to minimize crowding.

Going back to basics: What is a coronavirus?

This virus belongs to a family of viruses known as coronaviruses. Named for the crown-like spikes on their surfaces, they infect mostly bats, pigs and small mammals. But they mutate easily and can jump from animals to humans, and from one human to another. In recent years, they have become a growing player in infectious-disease outbreaks world-wide.

Seven strains are known to infect humans, including this new virus, causing illnesses in the respiratory tract. Four of those strains cause common colds. Two others, by contrast, rank among the deadliest of human infections: severe acute respiratory syndrome, or SARS, and Middle East respiratory syndrome, or MERS.

This new virus is called severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2. The disease it causes is called Covid-19. (The number stands for 2019, the year it emerged.)

How deadly is it?

The overall global mortality rate has ranged between 2% and over 4%, according to calculations of confirmed cases and deaths world-wide, which change daily. But the true rate won’t be known until epidemiologists can determine the denominator, meaning how many people have actually been infected. That number will include people who never had symptoms, or had a flulike illness but never got a test for Covid-19.

The mortality rate has differed by region, according to a report by an international mission of experts to China led by WHO. The mortality rate was 5.8% in the first several weeks in Wuhan, China, where the epidemic originated. But in other, less hard-hit areas of China, which had more time to prepare to care for patients, it was 0.7%. The rate in China has come down over time, the report said.

The overall mortality rate may be less than 1%, U.S. health officials suggested recently in the New England Journal of Medicine, if the number of asymptomatic or mildly ill cases is several times greater than that of reported cases.

That is still much deadlier than seasonal flu, which has a mortality rate of about 0.1%.

Covid-19 appears to be less deadly than the related pathogen—SARS, which erupted in China in 2002 and spread globally in 2003. SARS killed about 10% of the people it infected. Covid-19 is far less deadly than MERS or Ebola.

But this new virus spreads from one person to another more easily than SARS, some disease modeling and case studies suggest.

How Effective Are Masks?

The World Health Organization and other experts report that a mask's efficacy in social settings is inconclusive. But some health experts and mask makers say that properly used, the N95 respirator mask can guard against the new coronavirus.

Airborne Large droplets Surgical masks are highly effective against large airborne droplets. 39.9” Approx. distance large droplets can travel Small droplets These traditional masks are less effective with small droplets, as they can travel farther and in unpredictable paths affected by wind and other gusts. The droplets can be inhaled around the sides of the masks. Touch A coronavirus can also be transmitted by touching an object where airborne droplets have settled. The viruses can survive can survive for several hours or days depending on the kind of surface and conditions of humidity and temperature. Wearing a mask prevents direct contact with nose and mouth, and could protect a user who may have touched a contaminated area. Masks Frequently changing disposable masks, and washing your hands after, are important steps to avoid contamination from pathogens that cling to the outer surface. N95 masks offer more protection. But they only work if they fit properly, and aren’t suitable for children or people with facial hair. Surgical masks don’t offer full protection against airborne viruses. They don’t fully seal off the nose and mouth. Airborne Large droplets Surgical masks are highly effective against large airborne droplets. 39.9” Approx. distance large droplets can travel Small droplets These traditional masks are less effective with small droplets, as they can travel farther and in unpredictable paths affected by wind and other gusts. The droplets can be inhaled around the sides of the masks. Touch A coronavirus can also be transmitted by touching an object where airborne droplets have settled. The viruses can survive can survive for several hours or days depending on the kind of surface and conditions of humidity and temperature. Wearing a mask prevents direct contact with nose and mouth, and could protect a user who may have touched a contaminated area. Masks Frequently changing disposable masks, and washing your hands after, are important steps to avoid contamination from pathogens that cling to the outer surface. N95 masks offer more protection. But they only work if they fit properly, and aren’t suitable for children or people with facial hair. Surgical masks don’t offer full protection against airborne viruses. They don’t fully seal off the nose and mouth. Airborne Large droplets Surgical masks are highly effective against large airborne droplets. 39.9” Approx. distance large droplets can travel Small droplets These traditional masks are less effective with small droplets, as they can travel farther and in unpredictable paths affected by wind and other gusts. The droplets can be inhaled around the sides of the masks. Touch A coronavirus can also be transmitted by touching an object where airborne droplets have settled. The viruses can survive can survive for several hours or days depending on the kind of surface and conditions of humidity and temperature. Wearing a mask prevents direct contact with nose and mouth, and could protect a user who may have touched a contaminated area. Masks Frequently changing disposable masks, and washing your hands after, are important steps to avoid contamination from pathogens that cling to the outer surface. N95 masks offer more protection. But they only work if they fit properly, and aren’t suitable for children or people with facial hair. Surgical masks don’t offer full protection against airborne viruses. They don’t fully seal off the nose and mouth. Airborne Large droplets Surgical masks are highly effective against large airborne droplets. 39.9” Approx. distance large droplets can travel Small droplets These traditional masks are less effective with small droplets, as they can travel farther and in unpredictable paths affected by wind and other gusts. The droplets can be inhaled around the sides of the masks. Touch A coronavirus can also be transmitted by touching an object where airborne droplets have settled. The viruses can survive can survive for several hours or days depending on the kind of surface and conditions of humidity and temperature. Wearing a mask prevents direct contact with nose and mouth, and could protect a user who may have touched a contaminated area. Masks Frequently changing disposable masks, and washing your hands after, are important steps to avoid contamination from pathogens that cling to the outer surface. Surgical masks don’t offer full protection against airborne viruses. They don’t fully seal off the nose and mouth. N95 masks offer more protection. But they only work if they fit properly, and aren’t suitable for children or people with facial hair.

How easily does the virus spread?

Disease-modeling experts have estimated that on average, each infected person has transmitted the virus to about 2.6 others, though the range is between 1.5 and 3.5 and one recent report put that number as high as 5.7 for the epidemic in China. Those rates are higher than for some influenza viruses; some are lower than for SARS; and they are far lower than for measles, in which one infected person can transmit the virus to 12 to 18 other people.

Public-health experts caution that these estimates are preliminary, change over time and can be lowered by measures to prevent the virus from spreading.

Experts are debating how easily the virus is transmitted. The WHO-led mission of experts who visited China reported that clusters of transmission occurred largely in families, suggesting close contact. Other outbreaks suggest more widespread patterns.

What is the incubation period?

How the First Patients Fared

By Jan. 2 , Researchers identifed 41 novel coronavirus patients who had been admitted to the hospital in Wuhan, China.

MOST COMMON EARLY SYMPTOMS Fever 98% Cough 76 Aches, fatigue 44 HOW THE DISEASE PROGRESSED Avg. days since onset of symptoms 7 8 9 10.5 days Admitted to hospital Acute respiratory distress No. of patients 41 21 11 16 Admitted to intensive care unit Shortness of breath OUTCOMES* Out of 41 patients 28 Discharged from hospital 7 Still hospitalized 6 Died

People become ill between two and 14 days after infection, according to most estimates. One report described a person who became ill 27 days after infection. However, a study by researchers at Johns Hopkins University found that the median incubation period is 5.1 days, and 97.5% of those who develop symptoms will do so within 11.5 days.

Can you get infected after you have already had the disease?

That isn’t yet known. Sometimes a person is immune to a disease after an infection, but not always. Blood tests that reveal how many antibodies people who have recovered have will shed some light on the prospects for immunity.

Can you catch the virus from someone even before they have symptoms?

Several studies have now shown that people who have no symptoms, early symptoms or mild symptoms can transmit the virus to others. Scientists say this type of transmission might be common, and a possible explanation for why the virus spreads so quickly. They are studying it further. The CDC advises people to maintain a safe distance—6 feet or more—from others if Covid-19 is spreading in their communities.

Where did the new coronavirus come from?

The new virus likely came originally from bats, scientists say. It isn’t known exactly where or how it jumped to humans, though. Viruses from bats often infect another mammal first and then mutate to become more transmissible to humans. One hypothesis is that the intermediary animal for this new virus may be a pangolin, a small mammal sold in wildlife markets, prized for its meat and scales covering its body.

SHARE YOUR THOUGHTS Do you have questions about the risk of the new coronavirus from China? Join the conversation below.

Health officials believe the outbreak originated in a large animal and seafood market in Wuhan, China.

Of the first 41 cases, 27 had some exposure to that market, according to a report in the Lancet. But three of the first four people to become ill, on Dec. 1 and Dec. 10, said they had no contact with the market.

A study in the New England Journal of Medicine found that 55% of patients in Wuhan who became ill before Jan. 1 had a link to the market, compared with 8.6% of those who became ill after that point. Scientists say it will take some time to identify the exact source.

Is the virus mutating, particularly in a way that would make it more contagious?

While the virus has made some genetic changes—as RNA viruses are prone to do—none have made it deadlier or more contagious, according to experts.

Could imported goods carry the virus?

That is unlikely, the CDC says. Coronaviruses generally don’t survive that long on inanimate surfaces, according to the agency.

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—Talal Ansari contributed to this article.

Write to Betsy McKay at betsy.mckay@wsj.com