April 17, 2020 -- One of the great mysteries of the new coronavirus is why it causes only mild disease in most people, but turns fatal for others. In many cases, it seems the worst damage may be driven by a deranged immune response to the infection, rather than the virus itself.

In many of the sickest patients with COVID-19, their blood is teeming with high levels of immune system proteins called cytokines.

Scientists believe these cytokines are evidence of an immune response called a cytokine storm, where the body starts to attack its own cells and tissues rather than just fighting off the virus.

Cytokine storms are known to happen in autoimmune diseases like juvenile arthritis. They also occur during certain kinds of cancer treatment, and can be triggered by infections, like the flu. One study of patients who died of H1N1 influenza, for example, found that 81% had features of a cytokine storm.

Though the virus that causes COVID-19 has been circulating for only a few months, early research shows that like other infections, it, too, may cause this kind of catastrophic immune problem, and researchers say the size of the storm it triggers is gale-force.