As we see more coronavirus cases pop up across the U.S., here's a look at how it started, symptoms and what else you should know about COVID-19.

The new coronavirus that began as a handful of infections in central China has rapidly become a worldwide pandemic, shutting down entire cities, threatening the health of thousands and testing the strength of the global economy.

Almost 400,000 cases of the coronavirus have been confirmed worldwide across more than 160 countries and regions, and more than 17,000 people have died as of Tuesday, according to data from Johns Hopkins University. At the same time, almost 103,000 people worldwide have recovered from the virus.

As the rate of new cases in China continues to decline, more and more cases are being confirmed across the globe, with clusters in Italy and Iran. In the U.S., the death toll is creeping upward as new cases are cropping up across dozens of states.

Months into the outbreak, there are still more questions than answers. Here's what you need to know about COVID-19.

What are the symptoms of coronavirus?

Symptoms can range from mild to severe, and some people don't have any symptoms at all. The most common symptoms resemble the flu and include fever, tiredness and dry cough. Some people also develop aches and pains, nasal congestion, runny nose, sore throat or diarrhea.

About 1 in 6 people becomes seriously ill and develops difficulty breathing, according to the World Health Organization. If you experience fever, cough and shortness of breath, call your doctor.

Symptoms may appear anywhere between two to 14 days after exposure, with the average patient seeing onset at around five days, according to the CDC.

How many coronavirus cases are in the US?

There are more than 46,000 confirmed cases in the U.S., according to Johns Hopkins. Officials warn that many more people are likely infected. Of the confirmed cases, some are travel-related, some have spread person-to-person, and some were repatriated to the U.S. from Wuhan, China, and the Diamond Princess cruise ship.

Map of coronavirus cases in US

Cases have been reported in all 50 states and the District of Columbia, according to the Johns Hopkins data dashboard. 

How many people have died from the coronavirus in the US?

Almost 600 people have died after contracting the coronavirus in the U.S.

At least 87 of the deaths were in Washington state, where at least 29 of the patients died at EvergreenHealth Medical Center. At least 35 of the victims were associated with a nursing home in suburban Seattle, according to local health officials. At least one had previously been aboard the Diamond Princess cruise ship.

How many cases of coronavirus are there worldwide?

Here's a breakdown of worldwide numbers, as of Friday:

Almost 400,000 cases At least 17,000 people have died Almost 103,000 people have recovered Travel restrictions for US citizens

The State Department is urging Americans not to travel abroad at all. It issued a Level 4 advisory for travel abroad: "do not travel."

Last Wednesday, President Donald Trump announced the border between Canada and the U.S. would close for nonessential travel. The border closure goes into effect Saturday and will be reviewed after 30 days, officials said Friday. 

Trump announced Friday the U.S.-Mexico border would also close, but claimed trade would not be affected.

All foreign nationals from China, Iran and certain European countries are barred from entering the United States. This ban includes anyone who visited these countries within the 14 days prior to their U.S. trip. U.S. citizens are allowed to return home, but must fly into 13 designated airports and undergo "enhanced entry screening." View the full list of exceptions for entry here.

How to prepare for coronavirus

Take typical flu-season precautions: Avoid close contact with people who are sick. Don't touch your eyes, nose and mouth. Cover your cough. Stay home when sick. Clean household objects and surfaces. Wash your hands for at least 20 seconds.

If you're sick and want to prepare for a possible quarantine, here's a shopping list of items you should consider to avoid leaving your home. 

And no, you don't need a face mask unless you have COVID-19 and are showing symptoms. Buying up masks takes away precious materials from the health workers who need them most.

The White House recommends that you should:

Avoid social gatherings involving groups of more than 10. If someone in your house has testified positive, keep the entire household at home. Do not go to work or school. If you are an older person, stay home and away from other people. If you are a person with a serious underlying health condition, stay home and away from others. Avoid discretionary travel shopping trips and social visits. Avoid eating or drinking in restaurants bars, food courts. Use drive-through, pickup and delivery options. Who is most at risk of becoming very sick or dying?

Those at a higher risk of exposure to the virus include people who live in communities that are seeing sustained transmission, health care workers caring for COVID-19 patients and close contacts of patients.

As with seasonal flu, people at highest risk for severe disease and death include people aged over 60 years and those with underlying conditions such as hypertension, diabetes, cardiovascular disease, chronic respiratory disease and cancer, according to the WHO.

But Americans of all ages have faced serious health complications. New data from the Centers for Disease Control and Prevention show that among the roughly 12% of COVID-19 cases in the U.S. known to need hospitalizations, about 1 in 5 were among people ages 20 to 44.

Does coronavirus affect pregnancy?

It's unclear how the coronavirus may affect pregnant women. In general, pregnant women may experience changes to their body that could make them more susceptible to viral respiratory infections, according to the CDC.

In the case of SARS and MERS, pregnant women were more at risk for severe illness, and some experienced miscarriage and stillbirth, according to the CDC.

There is no evidence that the virus can be transmitted from an infected mother to her fetus, and, in a limited number of recent cases of infants born to mothers with COVID-19, none of the infants have tested positive for the virus, the CDC said.

Can kids get coronavirus?

Coronavirus in children appears to be rare, with about 2% of cases reported among people under 19 years old, according to the WHO study in China. An even smaller proportion of this age group developed severe (2.5%) or critical disease (0.2%), and just one person under 20 had died in China as of February.

How do you talk to children about coronavirus? Stay honest and simple to avoid anxiety.

Coronavirus vs. flu: How many people die from flu each year? 

In the U.S., influenza has caused 12,000 to 61,000 deaths annually since 2010, according to the CDC. 

So far this flu season in the U.S., there have been at least 38 million flu illnesses, 390,000 hospitalizations and 23,000 deaths from flu, according to the CDC. At least 149 of those deaths were in children.

Why is this being compared to the 1918 flu pandemic? What are the similarities and differences?

The Spanish flu, an H1N1 virus, emerged in the wake of WWI and spread across the globe between 1918 and 1919, infecting about a third of the world's population. An estimated 50 million people died, with about 675,000 deaths in the U.S., according to the CDC.

Unlike COVID-19, the 1918 pandemic had a high mortality rate among young, healthy people aged 20 to 40. While the Spanish flu's mortality rate was at least 2.5%, with some estimates as high as 10%, the mortality rate of COVID-19 appears to be closer to 3.4%, based on reported cases.

Like COVID-19, the main protections against the 1918 pandemic included isolation, quarantine, personal hygiene, use of disinfectants, and limitations of public gatherings, according to the CDC.

How did the coronavirus start?

Coronaviruses are a large family of viruses, named for their crown-like spikes. In rare cases, coronaviruses in animals have infected people, who spread the virus to other people. That's what happened with Middle East Respiratory Syndrome, or MERS, and Severe Acute Respiratory Syndrome, SARS, two far deadlier coronaviruses.

COVID-19, originally called the "novel coronavirus," was first detected in December 2019. The first infection was linked to a market in Wuhan, China, a city of 11 million people. It's still unclear how transmission unfolded, but there are several theories.

Some researchers believe that someone bought contaminated meat at the market, ate it, got sick and infected others. Others say the virus originated in bats, spread to an intermediary animal, and then to humans. Some researchers say pangolins may have been that intermediary host.

How is coronavirus spread? How long does coronavirus stay on surfaces?

The virus is spreading rapidly from person to person, and scientists are still learning more about how it spreads. According to the CDC, the virus spreads between people who are in close contact with one another (within about 6 feet) and through respiratory droplets, much like the common cold or flu.

See how coronavirus moved from one person to many in a matter of days

There's no evidence that the virus can be transmitted through food, and there is likely very low risk of spread from food products or packaging that are shipped over a period of days or weeks, according to the CDC.

It is, however, possible that a person can get the virus by touching a surface or object that has the virus on it and then touching their own face. A recent study by scientists in the U.S. found that viable virus could be detected up to three hours later in the air, up to four hours on copper, up to 24 hours on cardboard and up to two to three days on plastic and stainless steel.

USA TODAY analyzed three case studies that show how the virus was transmitted from person to person.

Is there a vaccine? Can coronavirus be cured?

A phase one vaccine trial is underway in the U.S., Trump said during a news conference with the coronavirus task force on Thursday.

Last month, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, estimated that a vaccine will be ready within 12 to 18 months.

Currently, there are no approved drugs or vaccines for coronaviruses, including COVID-19. Doctors can only treat the symptoms the viruses cause.

Chinese scientists have decoded the COVID-19 DNA and made it public, and several pharmaceutical companies in the U.S. and abroad are working to develop a vaccine.

What is the treatment for coronavirus?

Despite widespread rumors, social media reports and optimism surrounding the effectiveness of several existing drugs, so far there are no proven treatments for COVID-19, according to the U.S. Centers for Disease Control and Prevention.

Treatment consists of supportive care to help relieve symptoms and, for severe cases, care to support vital organ functions.

About 80% of people recover from the disease without needing special treatment, according to the WHO. For most patients, that means drinking plenty of fluids and resting, just as you would for the cold or flu.

Doctors and scientists are working furiously to find effective treatments but are cautioning the public not to self-medicate or hoard mentioned drugs not yet proven to work.

When COVID-19 treatments do arrive, they will likely fall into two categories, say experts. The first will be aimed at slowing down replication of the virus in patients early in the disease. The second will help stop the deadly autoinflammatory response in the lungs in its critical stage. Read more here. 

How long does coronavirus last?

Patients are infectious as long as they are "actively sick." But how long someone is actively sick can vary, according to the CDC.

Information about how long symptoms last is still evolving. But the February WHO study may give us some preliminary clues:

The median time from symptom onset to recovery is about two weeks for mild cases For patients with severe or critical disease, the median recovery time is three to six weeks Among patients who have died, the time from symptom onset to death ranges from two to eight weeks How do I get a coronavirus test?

If you're not quarantined on an air base or cruise ship, the only way to get a coronavirus test is through a health care professional. 

Call your doctor if you have a fever, develop virus symptoms, have recently traveled to an area with an ongoing spread of the virus or have come into contact with a person who is known to have it. You may be asked to wear a mask to the office.

If your doctor thinks you might have the coronavirus, he or she will contact the CDC or your local health department for instructions on testing.

Who can actually test for coronavirus?

As of March 19, 91 state and local public health laboratories in 50 states, the District of Columbia and Guam have successfully verified COVID-19 diagnostic tests and are offering testing, according to the CDC. Both academic labs and large private lab testing companies such as LabCorp and Quest Diagnostics are rolling out coronavirus tests after receiving FDA authorization.

While the CDC initially developed and mailed flawed testing kits, the agency said it fixed the glitch. Now, a new shortage of a key testing component is causing delays.

Meanwhile, a project funded by billionaire Bill Gates is set to issue at-home testing kits for people who fear they have been infected with the coronavirus.

How is it affecting our economy? Will the stock market ever recover?

The COVID-19 outbreak is taking a growing toll on the U.S. economy. The stock market has fluctuated wildly, and the number of Americans filing for unemployment benefits jumped last week as employers were forced to cut jobs following weakening demand caused by the pandemic.

Last Monday, the market had its worst sell-off in more than three decades. The Dow nosedived 2,999 points, or nearly 13%, to close at 20,188.52 – its second-worst percentage loss in history behind the "Black Monday" stock market crash in 1987.

U.S. stocks dropped Friday, capping their worst week since the height of the 2008 financial crisis. The Dow tumbled 913.21 points to close at 19,173.98, falling back below 20,000 after wild price swings over the past week. The S&P 500 fell 4.3% to end at 2,304.92.  For the week, the Dow dropped more than 17%, its worst one-week percentage drop since October 2008.

In an emergency response, the Federal Reserve cut interest rates on March 3 – the Fed's first rate cut between scheduled meetings since the depths of the financial crisis – and again on March 15, when the Fed cut short-term interest rates to zero.

What is the Trump administration doing about the epidemic? What is Mike Pence's role?

At the end of January, the Trump administration declared the coronavirus outbreak to be a public health emergency in the U.S. and quarantined Americans who had recently been to certain parts of China – the first quarantine order issued by the federal government in more than 50 years.

President Donald Trump put Vice President Mike Pence in charge of a task force that is coordinating the government's response to the outbreak. Trump also signed an emergency funding bill to combat the spread of coronavirus in the U.S. and help treat people who become ill. The $8.3 billion funding package amounts to one of the largest packages Congress will have passed to combat a global health crisis.

Trump also said he would use emergency authority to defer payments on federal loans to small businesses and to defer taxes for businesses hurt by coronavirus.

Treasury Secretary Steve Mnuchin said Thursday that the Trump administration wants checks of $1,000 per person and $500 per child to go out within three weeks of Congress passing a stimulus package. Mnuchin said he also wanted $300 billion to go toward small businesses for "hiring people, keeping people on the payroll, and if they do, there will be loan forgiveness." Another component involved $200 billion for "securing lending to airlines, and for other critical industries."

What's going on with cruises?

Dozens of confirmed cases have cropped up aboard cruise ships in recent weeks, and many other cases across the U.S. have been linked back to cruises.

In February, more than 700 passengers from the Diamond Princess became infected after the ship was held in quarantine off the coast of Japan. Of roughly 300 Americans evacuated from the ship and returned to the United States, 46 tested positive. Though most have ended their 14-day quarantine at U.S. military bases, some have remained in medical isolation as they recover.

Another ship, the Grand Princess, finally docked last week after being held off the coast of San Francisco. Of the more than 3,500 people on board, at least two passengers and 19 crew members have tested positive for the virus. Hundreds of Americans on board were expected to be taken to quarantine for two weeks.

Last week, major cruise lines including Carnival, Royal Caribbean, Norwegian, and Celebrity announced they would suspend sailing operations to and from U.S. ports for 30 days, Cruise Lines International Association.

What does it mean to quarantine versus isolate?

Isolation and quarantine are effective ways to help prevent the spread of disease, according to the CDC. Isolation separates sick people with a contagious disease from people who are not sick. Quarantine separates and restricts the movement of people who were exposed to a contagious disease to see if they become sick.

Follow Grace Hauck on Twitter @grace_hauck.

Contributing: Erin Richards, David Jackson, John Fritze, Michael Collins, John Bacon, Jorge Ortiz, Paul Davidson, Adrianna Rodriguez, Jayne O'Donnell, Ken Alltucker, Jessica Menton, Ryan Miller, Morgan Hines, David Oliver, Dawn Gilbertson, Curtis Tate, Jayme Deerwester.