THE GREAT INVADER: HOW COVID-19 ATTACKS EVERY ORGAN

We have underestimated and misunderstood COVID-19 since it first appeared.

And as we learn more, it's clear that COVID-19 can be more than just a respiratory disease. It's joined the ranks of other "great imitators" — diseases that can look like almost any condition.

It can be a gastrointestinal disease-causing only diarrhea and abdominal pain. It can cause symptoms that may be confused with a cold or the flu. It can cause pinkeye, a runny nose, loss of taste and smell, muscle aches, fatigue, diarrhea, loss of appetite, nausea and vomiting, whole-body rashes, and areas of swelling and redness in just a few spots.

In more severe disease, doctors have also reported people having heart rhythm problems, heart failure, kidney damage, confusion, headaches, seizures, Guillain-Barre syndrome, and fainting spells, along with new sugar control problems.

It's not just a fever and coughing, leading to shortness of breath like everyone thought at first.

This makes it incredibly difficult to diagnose and even harder to treat.

"This is a disease progression we have never seen for any infection that I can think of, and I've been doing this for a couple of decades," says Joseph Vinetz, MD, an infectious disease specialist at Yale School of Medicine.

How It Invades

When viral particles land in our eyes, nose, or mouth, "spike proteins" on the virus connect with a specific receptor, known as ACE2, on the surface of our cells, allowing entry. ACE2 receptors make a great target because they are found in organs throughout our bodies. Once the virus enters, it turns the cell into a factory, making millions and millions of copies of itself — which can then be breathed or coughed out to infect others.

In order to evade early detection, the coronavirus uses multiple tools to prevent the infected cells from calling out for help. The virus snips off distress signal proteins that cells make when they are under attack. It also destroys antiviral commands inside the infected cell. This gives the virus much more time to make copies of itself and infect surrounding areas before it is identified as an invader. This is part of the reason why the virus spreads before immune responses, like fever, begin.

Direct Attack

Many with mild or no symptoms are able to fend off the virus before it gets worse. These people may have symptoms only in the upper airway, at the site where they were first infected. But when someone's body can't destroy the virus at its entry point, viral particles march deeper into the body. The virus seems to take a few paths from there, either setting up camp in the lungs, fighting its way into the digestive tract, or doing some combination of both.

"There's clearly a respiratory syndrome, and that's why people end up in the hospital. Some people get a gastrointestinal illness with diarrhea, maybe some abdominal pain, which may or may not be associated with a respiratory illness," says Vinetz.