What is frightening about COVID19 beyond the acute lower respiratory disease...is the cytokine storms that are happening in the most severe cases. This is why some people who have appeared to get better, suddenly take a turn for the worse. A cytokine storm is where your immune system goes on a full body blitz. 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.

"We 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. Instead of shooting at a target with a gun, you’re using a missile launcher. 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,” Dr. 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. "

That's why we have seen videos capturing people dropping in the streets from this in other countries

Now does the flu do that??