In the early days of an infection, the novel coronavirus rapidly invades human lung cells. Those lung cells come in two classes: ones that make mucus and ones with hair-like batons called cilia.
Mucus, though gross when outside the body, helps protect lung tissue from pathogens and make sure your breathing organ doesn’t dry out. The cilia cells beat around the mucus, clearing out debris like pollen or viruses.
Frieman explains that SARS loved to infect and kill cilia cells, which then sloughed off and filled patients’ airways with debris and fluids, and he hypothesizes that the same is happening with the novel coronavirus. That’s because the earliest studies on COVID-19 have shown that
many patients develop pneumonia in both lungs, accompanied by symptoms like shortness of breath.
That’s when phase two and the immune system kicks in. Aroused by the presence of a viral invader, our bodies step up to fight the disease by flooding the lungs with immune cells to clear away the damage and repair the lung tissue.
When working properly, this inflammatory process is tightly regulated and confined only to infected areas. But
sometimes your immune system goes haywire and those cells kill anything in their way, including your healthy tissue.
“So you get more damage instead of less from the immune response,” Frieman says. Even more debris clogs up the lungs, and pneumonia worsens.
During the third phase, lung damage continues to build—which can result in respiratory failure. Even if death doesn’t occur, some patients survive with permanent lung damage. According to the WHO, SARS punched holes in the lungs, giving them “a honeycomb-like appearance”—and these lesions are present in those afflicted by novel coronavirus, too.
These holes are likely created by the immune system’s hyperactive response, which creates scars that both protect and stiffen the lungs.
When that occurs, patients often have to be put on ventilators to assist their breathing. Meanwhile, inflammation also makes the membranes between the air sacs and blood vessels more permeable, which can fill the lungs with fluid and affect their ability to oxygenate blood.
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In severe cases, you basically flood your lungs and you can’t breathe,” Frieman says. “That’s how people are dying.”
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Blood storm
Coronaviruses can also cause problems in other systems of the body, due to the hyperactive immune response we mentioned earlier.
A 2014 study showed that 92 percent of patients with MERS had at least one manifestation of the coronavirus outside of the lungs. In fact,
signs of a full body blitz have been witnessed with all three of the zoonotic coronaviruses: elevated liver enzymes, lower white blood cell and platelet count, and low blood pressure. In rare cases, patients have suffered from acute kidney injury and cardiac arrest.
But this isn’t necessarily a sign that the virus itself is spreading throughout the body, says Angela Rasmussen, a virologist and associate research scientist at Columbia University Mailman School of Public Health. It might be a cytokine storm.
Cytokines are proteins used by the immune system as alarm beacons—they recruit immune cells to the site of infection. The immune cells then kill off the infected tissue in a bid to save the rest of the body.
Humans rely on our immune systems to keep their cool when facing a threat.
But during a runaway coronavirus infection, when the immune system dumps cytokines into the lungs without any regulation, this culling becomes a free-for-all, Rasmussen says “Instead of shooting at a target with a gun, you’re using a missile launcher,” she says. That’s where the problem arises: Your body is not just targeting the infected cells. It is attacking healthy tissue too.
The implications extend outside the lungs. Cytokine storms create inflammation that weakens blood vessels in the lungs and causes fluid to seep through to the air sacs. “Basically you’re bleeding out of your blood vessels,” Rasmussen says. The storm spills into your circulatory system and
creates systemic issues across multiple organs.
From there, things can take a sharp turn for the worse. In
some of the most severe COVID-19 cases, the cytokine response—combined with a diminished capacity to pump oxygen to the rest of the body—can result in multi-organ failure. Scientists don’t know exactly why some patients experience complications outside of the lung, but
it might be linked to underlying conditions like heart disease or diabetes.
“Even if the virus doesn’t get to kidneys and liver and spleen and other things, it can have clear downstream effects on all of those processes,” Frieman says. And that’s when things can get serious.