How a mild case of the novel coronavirus can quickly turn deadly

Passengers from the Princess Cruises Grand Princess cruise ship are escorted to a charter plane at Oakland International Airport on March 10, 2020 in Oakland, California. Passengers are slowly disembarking from the Princess Cruises Grand Princess a day after it docked at the Port of Oakland. Some passengers will be flown to other states where they will quarantine for 14 days. The ship was held off the coast of California after 21 people on board tested tested positive for COVID-19 also known as the Coronavirus. less Passengers from the Princess Cruises Grand Princess cruise ship are escorted to a charter plane at Oakland International Airport on March 10, 2020 in Oakland, California. Passengers are slowly disembarking from ... more Photo: Justin Sullivan, Getty Images Photo: Justin Sullivan, Getty Images Image 1 of / 39 Caption Close How a mild case of the novel coronavirus can quickly turn deadly 1 / 39 Back to Gallery

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About four out of five people infected with the novel coronavirus experience mild or moderate symptoms. They likely will get a cough and a sore throat if the virus remains lodged in the nose and throat, but probably nothing worse.

The serious problems begin when the infection spreads to the lungs. When that happens, the disease can snowball from moderate to severe "very, very quickly," said Bruce Aylward, an assistant director-general with the World Health Organization.

Most of the time, COVID-19 starts in the nose before attacking the epithelial cells that line and protect the respiratory tract. If it hangs out there in the upper airway, patients will experience symptoms similar to the common cold. They'll be contagious but still mobile, still likely to go out in public.



In 10-15 percent of the moderate cases, the infection slides down the trachea and into the peripheral branches of the lungs, where it attacks two kinds of lung cells — the ones that make mucus, which protects against pathogens and lubricates the airway, and those with hair-like cilia.

University of Maryland School of Medicine associate professor Matthew B. Frieman, who studies coronaviruses, hypothesizes that COVID-19 hunts down cilia like SARS virus does, according to the National Geographic.

Cilia cells sweep debris such as pollen or viruses out of the lungs. When they're killed by an infection, the cilia slough off and fill patients' airways with fluids and debris.

At this stage, many patients develop non-life-threatening pneumonia in both lungs and shortness of breath. The body, alerted to the pathogenic invaders, triggers an immune system response: Immune cells flood the lungs to clear the organ and repair the damage.

In severe cases, however, the body's response can go overboard, killing not only virus-infected cells but healthy tissue as well, including mucus-producing cells and cilia. The friendly fire makes the pneumonia worse by leaving more debris clogging up the lungs.

What began as a mild infection seven or eight days previously has now escalated into severe pneumonia with acute respiratory distress syndrome (ARDS). The patient's condition rapidly deteriorates, requiring advanced life support to combat ARDS, according to a WHO report published in The Lancet. If they do survive, patients often suffer permanent lung damage.

"When you get a bad, overwhelming infection, everything starts to fall apart in a cascade," Dr. David Morens, senior scientific adviser to the director of the National Institute of Allergy and Infectious Diseases, told Bloomberg. "You pass the tipping point where everything is going downhill and, at some point, you can't get it back."

Healthy tissue damage from a too-robust immune system response can also render the lungs vulnerable to a secondary bacterial infection, especially from the antibiotic-resistant super bacteria that often inhabits hospitals and thrives around their mechanical ventilators. Septic shock, a severe and systemic infection caused by bacteria in your bloodstream, can result.

Secondary bacterial infections are especially dangerous because they can kill critical respiratory tract stem cells that enable tissue to rejuvenate, said Dr. Jeffery K. Taubenberger, who heads the viral pathogenesis and evolution section of the National Institute of Allergy and Infectious Diseases in Bethesda, Md.

Without the stem cells, "you just can’t physically repair your lungs," Taubenberger told Bloomberg.



ALSO: WHO declares COVID-19 a global pandemic

WHO's Aylward acknowledges that the elderly and those with pre-existing conditions are most at risk from the new coronavirus, but warns against soft pedaling the danger to others.

He discussed the issue on Canada's CBC News on Tuesday:

"What we've seen is people are cherry-picking data. So they look at the lowest possible case fatality rates and say, 'Oh look, it's like seasonal flu.' Or they pick, 'Oh it doesn't spread so fast because of this,' or 'Oh it only affects the older ones, I'm safe.' And none of those propositions are true.

"There's no reason to panic about this disease, but you have to be deeply concerned. This is not seasonal flu. It's an order of magnitude [with] tenfold — at least — higher mortality rate. Yes, it does strike vulnerable populations, but it can strike any age, let's be very, very clear — especially between your 30s and 60s and the older population."

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Mike Moffitt is an SFGATE Digital Reporter. Email: moffitt@sfgate.com. Twitter: @Mike_at_SFGate

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