https://www.nytimes.com/2020/04/01/health/coronavirus-cytokine-storm-immune-system.html

"When the body first encounters a virus or a bacterium, the immune system ramps up and begins to fight the invader. The foot soldiers in this fight are molecules called cytokines that set off a cascade of signals to cells to marshal a response. Usually, the stronger this immune response, the stronger the chance of vanquishing the infection, which is partly why children and younger people are less vulnerable over all to coronavirus. And once the enemy is defeated, the immune system is hard-wired to shut itself off.

“For most people and most infections, that’s what happens,” said Dr. Randy Cron, an expert on cytokine storms at the University of Alabama at Birmingham.

But in some cases — as much as 15 percent of people battling any serious infection, according to Dr. Cron’s team — the immune system keeps raging long after the virus is no longer a threat. It continues to release cytokines that keep the body on an exhausting full alert. In their misguided bid to keep the body safe, these cytokines attack multiple organs including the lungs and liver, and may eventually lead to death."

Koo