A doctor wearing a face mask looks at a CT image of a lung of a patient at a hospital in Wuhan, China.

AFP via Getty Images

Updated on March 17 at 6:43 p.m. ET:

Thousands
of people have now died from COVID-19 — the name for the disease caused
by the coronavirus first identified in Wuhan, China.

According to the World Health Organization, the disease is relatively mild in about 80% of cases.

What does mild mean?

And how does this disease turn fatal?

The first symptoms of COVID-19 are pretty common with respiratory illnesses — fever, a dry cough and shortness of breath, says Dr. Carlos del Rio,
a professor of medicine and global health at Emory University who has
consulted with colleagues treating coronavirus patients in China and
Germany. “Some people also get a headache, sore throat,” he says.
Fatigue has also been reported — and less commonly, diarrhea. It may
feel as if you have a cold. Or you may feel that flu-like feeling of
being hit by a train.

Doctors say these patients with milder symptoms should check in
with their physician to make sure their symptoms don’t progress to
something more serious, but they don’t require major medical
intervention.

But the new coronavirus attacks the lungs, and in
about 20% of patients, infections can get more serious. As the virus
enters lung cells, it starts to replicate, destroying the cells,
explains Dr. Yoko Furuya, an infectious disease specialist at Columbia University Irving Medical Center.

“Because
our body senses all of those viruses as basically foreign invaders,
that triggers our immune system to sweep in and try to contain and
control the virus and stop it from making more and more copies of
itself,” she says.

But Furuya says that this immune system response to this
invader can also destroy lung tissue and cause inflammation. The end
result can be pneumonia. That means the air sacs in the lungs become
inflamed and filled with fluid, making it harder to breathe.

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Del Rio says that these symptoms can also make it harder for the
lungs to get oxygen to your blood, potentially triggering a cascade of
problems. “The lack of oxygen leads to more inflammation, more problems
in the body. Organs need oxygen to function, right? So when you don’t
have oxygen there, then your liver dies and your kidney dies,” he says.
Lack of oxygen can also lead to septic shock.

The most severe
cases — about 6% of patients — end up in intensive care with multi-organ
failure, respiratory failure and septic shock, according to a February report from the WHO.
And many hospitalized patients require supplemental oxygen. In extreme
cases, they need mechanical ventilation — including the use of a
sophisticated technology known as ECMO
(extracorporeal membrane oxygenation), which basically acts as the
patient’s lungs, adding oxygen to their blood and removing carbon
dioxide. The technology “allows us to save more severe patients,” Dr.
Sylvie Briand, director of the WHO’s pandemic and epidemic diseases
department, said at a press conference In February.

Many of the
more serious cases have been in people who are middle-aged and elderly —
Furuya notes that our immune system gets weaker as we age. She says for
long-term smokers, it could be even worse because their airways and
lungs are more vulnerable. People with other underlying medical
conditions, such as heart disease, diabetes or chronic lung disease,
have also proved most vulnerable. Furuya says those kinds of conditions
can make it harder for the body to recover from infections.

“Of course, you have outliers — people who are young and otherwise previously healthy who are dying,” Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told NPR’s 1A
show. “But if you look at the vast majority of the people who have
serious disease and who will ultimately die, they are in that group that
are either elderly and/or have underlying conditions.”

Estimates
for the case fatality rate for COVID-19 vary depending on the country.
But data from both China and Lombardy, Italy, show the fatality rate
starts rising for people in their 60s. In Lombardy, for instance, the
case fatality rate for those in their 60s is nearly 3 percent. It’s
nearly 10 percent for people in their 70s and more than 16 percent for
those in their 80s.

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Del Rio notes that it’s not just COVID-19 that can bring on
multi-organ failure. Just last month, he saw the same thing in a
previously healthy flu patient in the U.S. who had not gotten a flu
shot.

“He went in to a doctor. They said, ‘You have the flu —
don’t worry.’ He went home. Two days later, he was in the ER. Five days
later, he was very sick and in the ICU” with organ failure, del Rio
says. While it’s possible for patients who reach this stage to survive,
recovery can take many weeks or months.

In fact, many
infectious disease experts have been making comparisons between this new
coronavirus and the flu and common cold, because it appears to be
highly transmissible.

“What this is acting like — it’s
spreading much more rapidly than SARS [severe acute respiratory
syndrome], the other coronavirus, but the fatality rate is much less,”
Fauci told 1A. “It’s acting much more like a really bad influenza.”

What
experts fear is that, like the flu, COVID-19 will keep coming back year
after year. But unlike the flu, there is no vaccine yet for the
coronavirus disease.