Scientists are racing to
unravel the mysteries of a new coronavirus that has infected thousands and sparked
global concern — triggering many
questions from researchers and the public alike. In this rapidly evolving
epidemic, many unknowns remain.

Here’s what we know so
far about the new virus, called 2019 novel coronavirus, or 2019-nCoV. We will
update these answers as more information becomes available.   

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

What is 2019-nCoV?

Coronaviruses are one of
a variety of viruses that typically cause colds. But three members of the viral
family have caused deadly outbreaks. Severe acute respiratory syndrome, or
SARS, Middle East respiratory syndrome, or MERS, and now 2019-nCoV cause more
severe disease, including pneumonia. This new coronavirus first emerged in Wuhan, China (SN: 1/10/20).

When did the outbreak start?

Chinese officials
notified the World Health Organization of a pneumonia-like disease with an
unknown cause in 44 patients on December 31, 2019. Initial reports tied the
disease —
now known to be 2019-nCoV — to a seafood
market in Wuhan, a city in Hubei Province. 

But the earliest cases may not be related
to exposure at the market
, researchers report January 24 in the Lancet. The earliest known patient with
the illness, who got sick December 1, was not exposed at the market, according
to the study.

So far, versions of the new virus isolated from patients in China
and other countries aren’t very different from one another. “This lack of
diversity fits with an origin in the human population in mid-November,” says
Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer
Research Center and the University of Washington in Seattle. 

Where did the virus come from?

Coronaviruses are
zoonotic, meaning they originate in animals and sometimes leap to humans.

Bats are often thought
of as a source of coronaviruses, but in most cases they don’t pass the virus
directly on to humans. SARS probably first jumped from bats into raccoon dogs
or palm civets before making the leap to humans. All the pieces necessary to
re-create SARS are circulating among bats, though that virus has not been seen since 2004
(SN: 11/30/17).

MERS, meanwhile, went from bats to camels before leaping to humans (SN: 2/25/14).
A paper published January 22 in the Journal of Medical Virology suggests
that the new coronavirus has components from bat coronaviruses, but that snakes may have passed the virus to humans. But many virologists are skeptical that snakes are behind the outbreak (SN: 1/24/20).

Current data suggest that
the virus made the leap from animals to humans just once and has been passing from person to person
ever since. Based on how closely
related the patient viruses are, animals from the seafood market probably
didn’t give people the virus multiple times as researchers originally thought,
Bedford says. If the virus leaped
from animals to humans multiple times, the researchers would expect a greater
number of mutations. Bedford and colleagues updated their conclusions and supporting
January 29 at

What are the symptoms of a 2019-nCoV infection?

An infection can cause fever,
cough and difficulty breathing, according to the U.S. Centers for Disease
Control and Prevention. These symptoms are similar to SARS, researchers report January 24 in the Lancet, though without intestinal symptoms or runny nose (SN: 1/24/20). Though many people with
2019-nCoV might experience mild symptoms, others can develop pneumonia.

Based on how MERS works, the CDC
reports that symptoms of 2019-nCoV may
appear from two to 14 days after exposure.
This is just an estimate at this point, though.

How infectious is the virus?

Researchers don’t yet
know for sure. But since 2019-nCoV has never infected humans before last year,
it’s likely that everyone is vulnerable to infection with this virus.

A virus’s potential infectivity
is described by its reproduction number called R0,
or R naught (SN: 1/24/20). It’s a
theoretical limit that researchers would expect to see when a disease-causing
organism hits a population where no one is immune, says Maimuna Majumder, a
computational epidemiologist at Boston Children’s Hospital and Harvard Medical
School. “In general, we don’t see transmission rates as high as the reproduction
number would suggest,” she says.

The number describes how
many other people are likely to catch a virus from an infected person. SARS,
for instance, has an R0 of 2.0 to 4.0 — each person who catches that virus has the potential to pass it on
to two to four others. Generally, viruses with reproduction numbers greater
than 1.0 may keep spreading if nothing is done to stop them. Outbreaks of
viruses with an R0 that falls at or below 1.0 may eventually peter

Several research groups
have been working to pin down an estimate based on outbreak information
available to them and by harnessing different methods, such as simulating
outbreaks or making assumptions about virus susceptibility, exposure and
infection rates. Others have used an approach that pulls data from current
cases and allows the researchers to describe what is happening in real time.

Current estimates vary,
and some have already been revised as health officials report new cases of
2019-nCoV. Majumder and her Harvard colleague Kenneth Mandl estimate the R0
falls between 2.0 and 3.1. Meanwhile, Jonathan Read of Lancaster
University in England and colleagues report a 2019-nCoV R0 value of 3.11, with a range from 2.39 to 4.13 on January 28 on But Christian
Althaus and Julien Riou, both of the University of Bern in Switzerland, posted
data to Github and on January 24 supporting their calculation that the new virus’s
reproduction number is about 2.2, with a range between 1.4 and 3.8.

R0 is a
tricky number to pin down, as the varying estimates reflect. It can also change
as control measures are put in place, suggesting that as more cases emerge,
these estimates will probably continue to shift. But currently the groups are
coming up with similar numbers, suggesting 2019-nCoV’s R0 is in the
same ballpark as SARS.

How does it spread?

Although initial Chinese
officials initially reported little evidence of human-to-human transmission, it is, in fact, how it’s now spreading (SN: 1/10/20).

Coronaviruses like SARS
and MERS —
and now 2019-nCoV — probably spread between people similar to other
respiratory diseases
, the CDC
says. Respiratory droplets from an infected person’s cough or sneeze can carry
virus to another person, something that generally happens between close

Because severe
coronaviruses infect deeper parts of the respiratory tract, compared with the
kinds of coronaviruses that cause colds, people tend not to be contagious until
they start to show symptoms, says Stanley Perlman, a virologist at the
University of Iowa in Iowa City.

In previous outbreaks,
“if somebody was going to get infected from [an] infected person, the virus had
to get up into the upper airway so it could spread,” Perlman says, which
wouldn’t happen until the patient was sick enough to start coughing.

Unlike SARS and MERS,
however, there have been some reports of people without symptoms spreading
2019-nCoV, Chinese officials announced January 26. And because people might be
infected and not show obvious symptoms, doctors should isolate patients and trace their contacts as soon as possible, researchers report
January 24 in the Lancet.

transmission is common for contagious viruses such as influenza or measles, but
would be a new feature for the types of coronaviruses that cause epidemics,
Perlman says.

It could make the
outbreak harder to control because such patients can spread disease without
signs that they’re sick themselves, making efforts such as airport screenings
less useful. But asymptomatic people have never been the major driver of
epidemics, Anthony Fauci, director of the National Institute of Allergy and
Infectious Diseases in Bethesda, Md., said in a news conference January 28.

How far has it spread?

So far, it’s unclear exactly how many cases there are, although
epidemiologists are attempting to calculate this number. 

But most of the thousands of people with confirmed diagnoses of the new virus are in China. But several other countries — 17 as of January 29 — have also confirmed isolated cases of the disease, many of whom had just returned from a trip to China.

A majority of countries outside of China have not reported
human-to-human transmission. But a man who had recently been to China transmitted the
virus to his son in Vietnam
, researchers confirmed January 28 in the New England Journal of Medicine. Four
people in Germany have also contracted
2019-nCoV after a visit from a colleague from China
, Reuters reports.
Those patients’ symptoms are currently mild, officials say. 

How deadly is the disease?

Coronaviruses usually
cause fairly mild illness, affecting just the upper airway. But the new virus,
like SARS and MERS, penetrates much deeper into the respiratory tract.
2019-nCoV is “a disease that causes more lung disease than sniffles,” NIAID’s
Fauci says. And damage to the lungs can make these illnesses deadly.

An analysis of 99
hospitalized patients, including the first cases from Wuhan, shows that 17
developed acute respiratory distress syndrome and 11 subsequently died from multiple organ failure, the Lancet reports January 29.   

The 2003 SARS outbreak killed nearly 10 percent of the 8,000 people
sickened (SN 3/26/03). MERS, a
disease that still circulates in the Middle East, is even more deadly, claiming about 30 percent
of the people it infects (SN: 7/8/16).
Right now, 2019-nCoV appears to be less virulent, with about a 4 percent mortality rate, the World Health Organization reported on
January 24. But that number is still a moving target as more cases are diagnosed, Fauci says.

“There almost certainly is asymptomatic
infection,” Fauci said January 29 at the 2020 ASM Biothreats meeting in
Arlington, Va. “We don’t know at what level yet.” Knowing that number affects
the calculation of how deadly the virus is. “Right now the denominator is
people who come to the hospital and are recognized. But with asymptomatic
infections, the denominator is going to be bigger and the mortality [rate] will
be less,” he said.

What is the situation in the United States?

There hasn’t yet been any
person-to-person transmission of the virus reported in the United States. As of
January 29, health officials have confirmed the coronavirus in five patients;
all had recently returned from Wuhan.

Twenty U.S. airports are
actively screening travelers from China for symptoms, according to the CDC.
Because of the relatively rapid release of information from China, countries
like the United States had time to put proper screening procedures in place,
giving them a leg up in keeping the virus from spreading, says Allison McGeer,
an infectious disease expert at Mount Sinai Hospital in Toronto who herself
contracted SARS in 2003 while taking care of patients.

What are the best ways to protect yourself?

The CDC recommends practices similar to preventing the spread of other respiratory
viruses, such as washing hands with soap and water for at least 20 seconds,
covering your cough or sneeze, and not touching your eyes, nose and mouth.

Personal actions, such
as wearing a mask, could also help contain the spread of the virus, though
experts say the evidence is inconclusive. “If you’re infected and you wear a
mask, you’ll shed less virus in to the air around you” and potentially reduce
the risk that others get infected, McGeer says. For uninfected people, the
effects of a mask are less clear, since they usually aren’t sealed tight around
the nose and mouth.

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