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While most cases of the new coronavirus are only mild or moderate, plenty are severe, with the disease having killed nearly 160,000 people worldwide as of Saturday, according to Johns Hopkins University.

What happens during a serious case of the disease?

First, the virus enters your respiratory tract

COVID-19 enters a person’s respiratory system through the mouth or nose. Under a microscope, spikes can be seen on the surface of the COVID-19 germ. These spikes are made of protein that allow the virus to latch on to a receptor in the body’s cells and begin duplicating, making the person sick.

If the virus stays in the upper respiratory tract, symptoms are generally mild

The new coronavirus can infect both the upper and lower respiratory tracts, unlike respiratory illnesses that might only infect one area or the other, health experts say.

A COVID-19 infection generally starts in the nose, health experts have said. The virus then attacks cells that line and protect the respiratory tract.

If the virus stays in the upper respiratory tract, the case will probably remain mild or moderate. Likely symptoms include fever and dry cough, and recovery in a week or two without medical care is typical.

If the virus moves to the lower respiratory tract, serious illness can occur

A lower respiratory infection has reached the lung and is what causes shortness of breath and tightness in the chest. The lungs and the respiratory tree, which helps bring oxygen into the lungs, can become inflamed, making it harder to breathe.

Lower respiratory tract infections can also lead to pneumonia. With pneumonia, the alveoli – where the blood exchanges oxygen and carbon dioxide – are infected and filled with fluid.

Symptoms of pneumonia include a high fever, sweating, shortness of breath, fatigue, a rapid heartbeat, the face turning blue and, in some cases, confusion.

Pneumonia can cause a patient to cough up greenish or yellow mucus. COVID-19 produces a dry cough, so a cough that yields mucus is a sign the illness has progressed to something more.

If any of these symptoms arise, it’s important to get medical help right away.

An ill-prepared immune system can attack the body, causing worse illness

In more serious cases of the illness, the body’s response to infection might be too strong, health experts say. It’s this aggressive reaction to COVID-19 – and not the virus itself – that causes serious illness and death, they say.

An immune system that has never experienced the new coronavirus doesn’t know how to respond. In scrambling to fight the infection, the immune system can end up destroying healthy cells and tissues.

This can leave the lower respiratory tract more susceptible to other bacteria and germs, causing a secondary infection, health experts say. The new infection can destroy cells that help repair the lungs.

“When you get a bad, overwhelming infection, everything starts to fall apart in a cascade,” 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.”

Further damage to the lungs may require you to get on a ventilator

If damage to the lungs worsens, they may lose their ability to deliver oxygen to other organs, leading to acute respiratory distress syndrome, or ARDS.

ARDS is treatable, but it can get worse quickly. Symptoms include rapid breathing, dizziness and sweating. ARDS further damages tissues and blood vessels in the alveoli, making breathing even more difficult. In many cases, a ventilator becomes necessary.

As the immune system works even harder to fight infection, it might release chemicals throughout the body, causing widespread inflammation and sepsis, an extreme response in which the blood pressure plummets and, in many cases, organs fail.

Dr. Carlos del Rio, a professor of medicine and global health at Emory University, told National Public Radio that these cascading effects are what can make COVID-19 so dangerous.

“The lack of oxygen leads to more inflammation, more problems in the body,” he said. “Organs need oxygen to function, right? So when you don’t have oxygen there, then your liver dies and your kidney dies.”

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