Answers to your questions on the new coronavirus

As the global health epidemic of COVID-19 spreads, many details remain unknown

The new coronavirus (round yellow objects in this scanning electron microscope image) is called SARS-CoV-2. The disease outbreak it has triggered began in China and has since spread to more than 70 other nations.


This story is being updated as news develops during this crisis. The last update was on Friday, March 18.

A new coronavirus has infected more than 200,000 people since December 2019. As it rapidly spreads across the planet, scientists and public-health experts are racing to limit the share of people it infects. To do that, they need to understand the new virus — called severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2. Yet many questions remain unanswered. Fortunately, details are starting to surface. 

Here’s what we know so far about the virus and the disease it causes. Check back regularly as we will be updating these answers as more information emerges. 

Do you have questions about the new coronavirus that you’d like answered? E-mail them to       

Some of the questions below include:

What is SARS-CoV-2?

The virus is a novel type of coronavirus. This is a family of germs that typically causes colds. But three members of this viral family can cause life-threatening disease, including pneumonia. They are severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and now SARS-CoV-2. The newest of these got its name because it is similar to SARS-CoV. 

The disease SARS-CoV-2 causes is coronavirus disease 2019, or COVID-19. Early in the outbreak, it was temporarily called 2019 novel coronavirus (2019-nCoV). 

Why are experts so worried about it?

Doctors still aren’t sure how contagious the virus is, nor how deadly. As a new coronavirus, it hadn’t infected people before the outbreak in China. Because it is new, people’s immune systems don’t have experience fighting the virus. So for now, everyone is susceptible to infection. And it can spread rapidly and widely.

Scientists and public health officials particularly worry about people in high-risk groups. So far, older adults and people with underlying health conditions (such as heart or lung disease) appear to face a higher risk of severe disease.

In several countries, there has been a sudden, big spike in cases. If that happens in the United States, COVID-19 patients could have to compete for hospital space with other sick people. Too big a spike could overwhelm hospitals.

Some experts still see a chance to limit the outbreak and prevent the virus from taking root in the population, as flu and other respiratory diseases have. Seasonal influenza, for instance, causes yearly epidemics. Because flu infects millions of people — and kills about a tenth of one percent of all it sickens — tens of thousands of people can die from it every year. By tracing contacts and isolating sick patients, experts hope to keep COVID-19 from doing this.   

So how deadly is the new virus?

The best data, to date, come from passengers exposed to the virus while on the Diamond Princess cruise ship. After one of them tested positive for SARS-CoV-2, the ship was quarantined at sea off Japan. In the process, it became a natural data lab. Nearly everyone on board was tested. Few cases of infection were missed. 

Those data now suggest that the disease’s true fatality ratio in China is about 0.5 percent. That is far less than a 3.4 percent death rate that had been cited by the World Health Organization. The WHO’s number had come under fire because the true number of people infected with the virus worldwide is not known.   

But death rates can vary by region, depending on how the infected patients were treated, the number of vulnerable people who were exposed and more. Officials say the death rate therefore will likely change as the outbreak continues and hits new populations.

As of March 18, more than 212,000 people have become infected globally, and more than 8,700 have died.

For comparison, the 2003 SARS outbreak was far more deadly. But because it sickened fewer people overall, its death toll never rose above 774. The virus that causes MERS, a disease that still circulates in the Middle East, is even more deadly. It kills one in every three people it infects — or 866 people so far.    

The overall deadliness of COVID-19 may not be known for a while. Researchers will need to find out how many people have been infected but showed no symptoms (or had such mild symptoms they didn’t get tested). 

Who’s most at risk? What about young children? 

The elderly are most vulnerable, based on an analysis of about 44,000 cases of COVID-19 in China. Older people, especially those with heart disease and other conditions, are more likely to die. Middle-aged and elderly adults are most likely to get the virus. For some reason, children and teenagers seem to rarely become seriously ill with COVID-19. Yet even children with mild illness may still spread the virus.

Another study was published March 16 in Pediatrics. It describes the disease in 2,143 children in China. About half lived in Hubei Province, the epicenter of the pandemic. Compared with adults, these children generally had milder cases. It’s unknown why most kids aren’t getting as sick as adults are. 

But children weren’t wholly protected. An estimated 6 in every 100 kids had severe or critical disease. Infants and preschoolers generally had more severe illnesses than older kids, the team found. Their symptoms including trouble breathing. The researchers report that one 14-year-old boy even died.

What are the symptoms?

People with COVID-19 often have a dry cough and sometimes shortness of breath. The broad majority of patients will also spike a fever, according to reports on patients in China. 

One tricky thing is that these symptoms also apply to the flu. And right now it’s flu season in the United States. So most people with those symptoms likely do not have COVID-19. 

Respiratory illnesses caused by other types of viruses (such as rhinoviruses and enteroviruses) usually do not cause fevers, notes Preeti Malani. She’s an infectious disease specialist at the University of Michigan School of Medicine in Ann Arbor. Colds often include a runny nose. So far, however, COVID-19 has not left many people with drippy noses. 

Though many people infected with SARS-CoV-2 will probably have mild symptoms, others may develop life-threatening pneumonia. Here, tiny air sacs in the lungs can become inflamed and fill with fluids, even pus. Symptoms of pneumonia can include chest pain when breathing or coughing, fever, a phlegmy cough, shortness of breath and more. 

How do people die from COVID-19?

Coronaviruses usually cause fairly mild illness in the nose and throat. But as with SARS and MERS, the new virus works its damage much deeper in the respiratory tract. SARS-CoV-2 leads to “a disease that causes more lung disease than sniffles,” explains Anthony Fauci. He directs the National Institute of Allergy and Infectious Diseases in Bethesda, Md. It’s that damage to the lungs that can turn these illnesses deadly.

Patients with COVID-19 generally die from breathing difficulties and the failure of several organs. Those organs fail partially because of what the virus does, but also because of how the body’s immune system attacks the infection. The virus that causes COVID-19 attacks cells within the respiratory tract, especially the lungs. As these cells die, they fill the lungs’ airways with fluid and debris. Meanwhile, the virus hijacks living cells there to replicate. All of this overwhelms the lungs, making it hard to breathe. 

The presence of dying cells and a replicating virus also sparks the immune system to react to the germs. Immune cells flood the lungs. There, they attempt to repair damaged tissues and wipe out the virus. This immune response usually is well controlled. However, it can sometimes go berserk and damage healthy cells as well as attempting to remove dying ones. A flood of signals from the immune system, called a cytokine (SY-tuh-kyne) storm, can damage tissues so badly that the lungs and other organs can simply give out.

How long does it take for symptoms to show up?

The time it takes for symptoms to show up is estimated to usually be around four to five days. But it may be as short as two and as long as 14 days. This delay is known as the “incubation period.” Older people may have a slightly longer incubation period. One preliminary study found that people over 40 show symptoms after six days. In contrast, younger people may show symptoms after just four days. These data were posted February 29 on

How long are people contagious?

Researchers are starting to get hints of just when patients are most contagious. Infected people may shed infectious virus both before and after they have symptoms. That’s according to one study posted March 8 at It describes nine people who contracted the virus in Germany. This study finds that people are mainly contagious before they have symptoms and in the first week after showing signs of disease.

Patients produced thousands to millions of viruses in their noses and throats, about 1,000 times as much virus as produced in SARS patients, Clemens Wendtner directs infectious disease and tropical medicine at Munich Clinic Schwabing. It’s a teaching hospital. There, he and his colleagues found that a heavy load of viruses may help explain why the new coronavirus is so infectious.

Scientists identified these nine people some time after they had been exposed to the virus. So they don’t know for sure when exactly people begin giving off the virus.

After the eighth day of symptoms, the researchers could still detect the virus’s genetic material, called RNA, in patients’ swabs or samples. At that point, however, they could no longer find infectious viruses. That’s an indication that antibodies made by the body’s immune system are killing viruses that get out of a patient’s cells, Wendtner says.

How does the disease spread?

Coronaviruses like SARS and MERS — and now SARS-CoV-2 — probably spread between people in ways similar to other respiratory diseases, says the U.S. Centers for Disease Control and Prevention. Germy droplets from an infected person’s cough or sneeze can infect someone standing almost two meters (6.6 feet) away. 

Touching surfaces covered with droplets and then touching your face can also spread the virus. New research suggests that this virus remains viable longest on plastic and stainless steel. There it can be detected for two to three days, although infectivity drops substantially after 48 hours. That’s according to a study posted March 9 at and later published in NEJM. On cardboard, the virus lasts for 24 hours. “Live” virus also lasted three hours or more in the air. So even walking through a room where someone had coughed may pose the risk of infection, these data suggest.

Can people who have had the virus be reinfected?  

Not likely, experts say.

There have been some reports of patients testing positive for the virus after they have recovered. They may even appear to get sick again. It’s likely, however, that the virus survived in the body longer than expected. Or people who appeared to recover just relapsed after seeming to get well. These results could also reflect issues with the current diagnostic test, which isn’t sensitive enough to always pick up low levels of virus in an infected person. 

“I don’t think that reinfection is that likely,” says Angela Rasmussen. She’s a virologist at Columbia University in New York City. But studying the disease in other animals, such as mice or nonhuman primates, could help determine whether the virus can lead to reinfections, she says.

One small study in rhesus macaques found that the animals couldn’t be reinfected with the coronavirus, at least in the short term. Those findings come from a report posted March 14 at The monkeys developed antibody responses against the virus. That is what likely protected them from getting infected when they were exposed again 28 days after their first exposure. It’s still unclear, however, how long immune responses against the virus last.

Is the virus spread by people with no symptoms?

Unlike SARS and MERS, there now is ample evidence that people showing no symptoms — or very mild ones — can spread the new virus. Symptom-free spread is common for a number of contagious viruses. These include influenza and measles. It would, however, be something new for the types of coronaviruses that cause epidemics.

How big of a problem is symptom-free spread?

Right now, no one knows. Researchers would need to understand how many people, in total, have been infected. To learn this, they need a test to identify people who have developed antibodies against the virus. That would confirm they had been infected, even if body had cleared out the virus. So far only Singapore has done such tests.

But mild cases of COVID-19 that go unrecognized are fueling the coronavirus pandemic. That’s the finding of a study in Science based on data from the early days of the outbreak in China. Undocumented cases — those in people with mild or no symptoms — accounted for almost nine in every 10 infections. The good news: Those undetected cases were apparently only half as infectious as known cases. Symptom-free spread could make the epidemic very hard to control because such patients can spread disease with no signs that they’re sick.

How far has the disease spread? 

As of March 18, the virus has infected more than 212,000 people in at least 157 countries and territories.

From March 1 to March 2, nine times as many cases were reported outside China as inside. That’s partly because the number of daily cases reported in China is going down. And that’s a sign that containment efforts there are continuing to slow the outbreak. At the same time, cases in other countries are going up. And as of March 15, the number of COVID-19 cases in countries outside of China has surpassed the number of cases there. 

“Europe has now become the epicenter of the pandemic,” said WHO director-general Tedros Adhanom Ghebreyesus at a March 13 news conference. Apart from China, he said, Europe has more reported cases and deaths than the rest of the world combined.”

Cases of COVID-19 in Italy, for instance, have skyrocketed. More than 11,000 new cases were documented there from March 6 to March 13. The entire country is now on lockdown. Shops have closed and there have been major travel restrictions.

Meanwhile, officials in South Korea have focused on aggressive monitoring. There, the number of new cases is declining. Officials in that country have set up more than 500 coronavirus-testing sites around the country. Throughout mid-March they were screening some 10,000 people per day.  

How many undetected cases are out there?

No one knows for sure. One reason: There aren’t enough test kits to test everyone who might be infected. Another reason is that people may be infected with the virus but show no symptoms or very mild ones. Those people may, however, still be able to infect others.

“There’s really no doubt that there are many undetected cases,” says Erik Volz. He’s a mathematical epidemiologist at Imperial College London in England.

Why do we care about undetected cases?

Undetected cases matter because they may seed outbreaks when travelers become infected and then carry them to other places, says Katelyn Gostic. She’s a researcher in Illinois at the University of Chicago. There, she studies the spread of infectious diseases. But even the best efforts to screen airline passengers for COVID-19 likely will miss about half of cases. That’s what she and her colleagues reported February 25 in eLife.

Those missed cases at airports “are not due to correctable mistakes,” Gostic says. It’s not that sick travelers are trying to avoid detection. And it’s not that screeners are bad at their jobs. “It’s just a biological reality,” she says, that most infected travelers won’t realize they’ve been exposed and won’t show symptoms. 

That is true for most infectious diseases. But the share of COVID-19 cases with mild or undetectable disease poses a big challenge. These people may see no cause to seek medical attention and get tested. Yet they may be able start epidemics in new places. 

Is containment of the virus still possible?

Tedros Adhanom Ghebreyesus is director-general of the World Health Organization. It’s based in Geneva, Switzerland. Despite the rising case numbers outside China, “containment of COVID-19 is feasible and must remain the top priority for all countries,” Tedros said at a March 2 news briefing. “With early, aggressive measures,” he said, “countries can stop transmission and save lives.”

Limiting travel, closing public transportation and banning mass gatherings (such as concerts) can reduce spread of a virus. Also, “there are no-brainers like school closure,” that might help limit spread of this virus, says Volz at Imperial College. Children aren’t suffering much severe illness. Still, they may get infected and spread the virus to others.

It’s unclear how long such measures would have to be in place to be useful. It likely will depend on what is happening in the affected area. But some regions have closed down schools, including countries like China, Japan and France, as well a few U.S. states. 

What’s the situation in the United States?

As of March 18, U.S. health officials have confirmed the new coronavirus in 7,708 people across all 50 states and territories, with 120 deaths.

Officials announced the first COVID-19 case in the United States linked to travel on January 21. On February 26 and 28, U.S. health officials reported two women in California had been infected. What was special here: Neither woman had traveled to affected areas nor been exposed to someone known to have the disease. Such cases are examples of what is known as “community spread” of an infection. Officials have since identified a growing number of community-spread cases in Washington state. This suggests the virus is spreading locally there. As of March 18, at least 1,012 patients there had tested positive for the virus — including 52 of the U.S. deaths.

In fact, the virus may have been circulating in Washington state for at least six weeks. That estimate comes from a genetic analysis of virus samples from two patients in the state conducted by a team of researchers that included Trevor Bedford. He’s an evolutionary biologist at the Fred Hutchinson Cancer Research Center and the University of Washington in Seattle. The team now estimates that a few hundred people in Washington may be infected, Bedford said March 2 on Twitter. If nothing is done to stop them from sharing it with others, the size of the Washington outbreak could double every week, Bedford said. 

In the wake of steadily rising case numbers, health officials have put in place new “social distancing” measures. These include advising people to avoid gatherings of more than 10 people through at least early April. Some states have shut down bars and restaurants. And on March 17, officials in the San Francisco Bay area enacted a shelter-in-place order. It affected close to 7 million people.

What can I do to prepare?

Practicing good hygiene is the most important way to protect yourself. Tips from the infectious disease experts include washing hands with soap and water or using alcohol-based sanitizers. We have published a top 10 list of these tips. If you think you are sick, stay home and avoid traveling. 

The U.S. Centers for Disease Control and Prevention also recommends having a plan in place for how you and your family will get by if and when all or part of your family must stay home from work or school.  

Does hand sanitizer actually work?

Though washing your hands with soap and water is best, hand sanitizers also work.

When a single virus leaves an infected cell, it takes part of that cell’s membrane with it. This membrane forms a protective envelope around the virus. But the alcohol in hand sanitizer can disrupt this envelope. That essentially kills the virus.     

Why aren’t masks recommended for protection?

Surgical masks are designed to hold germs in, not keep them out. 

If a sick person wears a surgical mask, the fabric will catch germy droplets that get coughed or sneezed out. That can prevent the virus from getting onto surfaces that other people might touch. 

Such masks are not, however, designed to protect healthy people from viruses in the environment. Surgical masks don’t fit perfectly around the face. This leaves gaps on the sides. Many people also don’t wear them properly (such as leaving their nose exposed while covering their mouth). 

For more details, on March 4, JAMA Network published a whole webpage for the public on masks and how they should — or should not — be used during an epidemic such as COVID-19.

What should I do if I think I have COVID-19?

If you have a fever and respiratory symptoms, call your medical provider ahead of time, says Malani, the infectious-disease expert. Let them guide you on what step to take next. “This is not something that you can just walk into an urgent care [clinic] and easily get tested,” she says. 

Local health departments, with help from doctors, have been asked to figure out whether someone should be tested for coronavirus. Testing initially occurred at the Centers for Disease Control and Prevention in Atlanta, Ga. But the Association of Public Health Laboratories expects that more labs will begin testing soon.

It’s important to remember that for most people, the risk of getting severely ill appears to be fairly low. But “even though individual risk may be low, there’s still a need to take the situation seriously,” says Gostic at the University of Chicago. Even if you face a low risk of severe disease, if you get sick, you risk spreading COVID-19 to someone at high risk of serious illness. 

How do doctors test for the virus?

The World Health Organization suggests that doctors take multiple samples for testing. These should include swabs of the nose and throat together with blood and with phlegm from the lower respiratory tract.

In the lab, researchers look for genetic evidence of the virus. They do this using a technique called RT-PCR. (That’s short for reverse transcription polymerase chain reaction.) If virus is found, the technique will make copies of its RNA — the virus’s genetic code — that is unique to SARS-like coronaviruses. Where the tests come back positive, researchers will do more analyses to pin down whether SARS-CoV-2 was the virus present. The method relies on patients being sick enough that they had high amounts of the virus at the time of collection. So not everyone who is infected will have a positive test. 

Initial testing in the United States was limited to people with a travel history to Wuhan, China, or to people who had come into contact with someone who had. The U.S. Centers for Disease Control and Prevention recently expanded its testing criteria after learning of the first cases of community spread. On March 12, the CDC announced it will allow much wider testing. 

The first CDC diagnostic kits for SARS-CoV-2 had been flawed. That limited the screening of patients by local and state labs. As of March 3, federal officials said they expected within a few days the United States would have the capacity to run some one million tests. In fact, by March 13, the U.S. total was only around 14,000, according to the CDC.

Where did the virus come from?

Coronaviruses are zoonotic. That means they had been in animals and then leaped to people. Such diseases may reach people when those animals are handled, kept as pets or prepared to be eaten. In the case of SARS-CoV-2, experts suspect bush meat — wild animals eaten as human food — may be the initial source.

Bats are known to host many coronaviruses. In most cases, however, they don’t pass the virus directly on to people. SARS probably first jumped from bats into raccoon dogs or palm civets before making the leap to humans. (People in Asia sometimes eat civets, bats and other animals.) MERS went from bats to camels before leaping to humans. 

A paper published January 22 in the Journal of Medical Virology suggests that parts of the new coronavirus appear to have come from bat coronaviruses — but that snakes then may have passed the virus to people. Many virologists, however, doubt that are skeptical of that. Other analyses have proposed that unusual mammals known as pangolins might be the source. But coronaviruses from pangolins are not as closely related to SARS-CoV-2 as are bat viruses. So the mystery is far from solved. 

Current data suggest that the virus made the leap from animals to people just once. Since then, it has been spreading from person to person. 

Why does knowing the virus’s origin matter?

Pinpointing the source of the virus is a step toward protecting people from coming into contact with more infected animals, and possibly starting another outbreak. 

Can pets get sick?

Scientists have confirmed the new coronavirus in a pet dog in Hong Kong. It had low levels of the virus, which the animal may have gotten from its owner. It’s only a single case, so fa. For now, there is no evidence that the virus can make pets sick (show symptoms) or pass it to people or other animals.

Several types of coronaviruses can infect animals. In some cases, it can make them ill. So the U.S. Centers for Disease Control and Prevention (CDC) advises avoiding contact with pets if you are sick (or wearing a face mask if you must).

The dog’s tests are only weakly positive. So it may be a dead-end host, meaning the virus is not reproducing at high enough levels in the dog to be spread. Tests for the new virus rely on detecting small bits of the germ’s genetic code. It does not measure whether that virus was infectious enough to infect another. Researchers are now analyzing the genetic blueprint of the dog’s virus to learn more about it and where it came from.

“We need to establish quite clearly what part animals might play in transmission. That is unknown,” says Michael Ryan. He’s the executive director of the World Health Organization’s Health Emergencies Programme. Ryan was speaking about the dog case during a March 5 news conference. Keep in mind that animals getting infected is “not an unusual or unprecedented finding. It happens regularly with emerging diseases,” he said.

The CDC also recommends that people traveling to China avoid animals.

When will it end?

That’s a tough question, experts say. It’s not looking likely that control efforts will stop the COVID-19 epidemic and cause it to disappear, as SARS did. Like flu or the common cold, it may just begin circulating permanently in humans. Right now, no one knows if the virus might become seasonal, like the flu. 

Though not yet a pandemic, it’s possible that COVID-19 could be declared one. Pandemics are usually defined as the worldwide spread of a new disease. And as the top global health agency, the World Health Organization, or WHO, would be the first to make the call on whether COVID-19 has become one. An outbreak that is large but confined to a specific region is known as an epidemic. Once the epidemic spreads to two or more continents — and shows sustained, person-to-person transmission — the WHO may declare it a pandemic. The agency’s decision relies chiefly on the disease’s spread, not its severity.

Tedros Adhanom Ghebreyesus is the WHO’s director-general. “Does this virus have pandemic potential? Absolutely,” he said in a February 24 news conference. Are we there yet? “From our assessment,” Tedros said, “not yet.”

Erin Garcia de Jesus, Tina Hesman Saey, Aimee Cunningham and Jonathan Lambert contributed to reporting of this story.  

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