Its true fatality rate isn't yet known, but it seems 10 times higher than the flu, which kills hundreds of thousands around the world each year, the United States' top infectious disease expert told lawmakers last week.

Most people have had mild to moderate illness and recovered, but the virus is more serious for those who are older or have other health problems.

That's a huge number, said Dr. Tom Frieden, a former director of the Centers for Disease Control and Prevention who now heads a global health organization. In the U.S., 60% of adults have at least one underlying health condition and 42% have two or more.

"There's still a lot that we don't know" about the virus and disease it causes, COVID-19, he said.

HOW IT SPREADS

Most spread is from droplets produced when an infected person coughs, which are inhaled by people nearby. Transmission from touching contaminated surfaces hasn't been shown yet, though recent tests by U.S. scientists suggest it's possible -- one reason they recommend washing your hands and not touching your face.

The virus can live in the air for several hours, up to 24 hours on cardboard and up to two to three days on plastic and stainless steel. Cleaning surfaces with solutions containing diluted bleach should kill it.

"While we are still learning about the biology of this virus, it does not appear that there is a major risk of spread through sweat," said Julie Fischer, a Georgetown University microbiologist. The biggest concern about going to the gym is infected people coughing on others, or contaminating shared surfaces or equipment, she said. Consider avoiding large classes and peak hours and don't go if you're coughing or feverish, she suggests.

The risk of virus transmission from food servers is the same risk as transmission from other infected people, but "one of the concerns in that food servers, like others facing stark choices about insurance and paychecks, may be pressured to work even if they are sick," she said.

HOW FAST DOES IT SPREAD?

Each infected person spreads to two or three others on average, researchers estimate. It spreads more easily than flu but less than measles, tuberculosis or some other respiratory diseases. It is not known if it spreads less easily among children, but fewer of them have been diagnosed with the disease. A study of 1,099 patients in China found that 0.9% of the cases were younger than 15.

WHAT ARE THE SYMPTOMS?

Most people get fever and cough, sometimes fatigue or shortness of breath, and recover after about two weeks. About 15% develop severe disease, including pneumonia, Chinese scientists reported from 45,000 cases there. Symptoms usually start slowly and often worsen as the illness goes on.

In a  report last week on the first 12 patients in the U.S., seven were hospitalized; most had underlying health problems and got worse during the second week of illness.

In China, slightly more males have been diagnosed with COVID-19 than females, which might be because roughly half of Chinese men smoke but only 5% of females do, Frieden said.

Children seem to get less sick -- a report on 10 in China found that fevers tended to be milder and they lacked clear signs of pneumonia.

WHAT DOES IT FEEL LIKE?

Some cruise ship passengers described symptoms similar to the common cold or flu.

"It's been a 2 on a scale of 10," said Carl Goldman, who was hospitalized in Omaha, Nebraska, after flying home.

However, a Chinese postgraduate student described going to the hospital twice after her symptoms worsened, and feeling "a heavy head while walking, unable to breathe, and nauseous."

WHAT'S THE TEST LIKE?

The CDC recommends at least two swabs -- nose and throat. Samples are sent to labs that look for bits of viral genetic material, which takes roughly 4 to 6 hours. Altogether, it can take several days to ship a sample and get results back.

It's been taking two to three days, and "we are working really hard to see if we can shorten that time" by developing an in-house test, Dr. Aimee Moulin of the University of California, Davis said Thursday in a conference call held by the American College of Emergency Physicians.

Some areas have opened drive-thru sites for testing, which could reduce exposure to health workers and other patients or the public.

WHEN IS THE VIRUS MOST CONTAGIOUS?

The average time from exposure to developing symptoms is five to six days, but can be up to two weeks. Tests have found high amounts of virus in the throats and noses of people a couple days before they show symptoms.

Signs of virus also have been found in stool weeks after patients recover, but that doesn't mean it's capable of causing illness, scientists warn.

"The virus can be degraded," said Robert Webster, a St. Jude Children's Research Center virus expert. "It's not necessarily infectious virus at all."

HOW DEADLY IS IT?

That won't be known until large studies are done to test big groups of people to see how many have been infected and with or without symptoms.

Scientists have estimated the fatality rate from less than 1% to as high as 4% among cases diagnosed so far, depending on location.

Flu kills about 0.1% of those it infects, so the new virus seems about 10 times more lethal, the National Institutes of Health's Dr. Anthony Fauci told Congress last week.

The death rate has been higher among people with other health problems -- more than 10% for those with heart disease, for example. In the U.S., 30 million have diabetes, more than 70 million are obese and nearly 80 million have high blood pressure.

CAN INFECTED PEOPLE WHO RECOVER GET IT AGAIN?

It's not known. A few reports from China say some people had COVID-19, recovered and then fell ill again. It's unclear if that's a relapse, a new infection, or a case where the person never fully recovered in the first place.

Scientists at the at the Fred Hutchinson Cancer Research Center in Seattle say the 30,000-letter genetic code of the virus changes by one letter every 15 days. It's not known how many of these changes would be needed for the virus to seem different enough to the immune system of someone who had a previous version of it for it to cause a fresh infection.

Fauci told Congress on Thursday that it was unlikely that someone could get reinfected.

"We haven't formally proved it, but it is strongly likely that that's the case," he said. "Because if this acts like any other virus, once you recover, you won't get reinfected."

WILL IT GO AWAY IN THE SUMMER?

Flu fades each spring and the new virus may do the same, Fauci said last week in a podcast with a journal editor.

"I am hoping that as we get into the warmer weather we will see a decline that will give us a chance to get our preparedness up to speed," Fauci said.

But that, too, is far from certain. "We have to assume that the virus will continue to have the capacity to spread, and it's a false hope to say yes, it will just disappear in the summertime like influenza," said Dr. Michael Ryan, the World Health Organization's emergencies chief.

Flu viruses also mutate quickly, requiring new vaccines to be made each year. If the coronavirus follows suit, Frieden said, "It could become a virus that circulates around the world for many years to come."