Our understanding of Covid-19 as a disease has evolved rapidly over the course of the pandemic. While many aspects of Covid-19—from its symptoms to its duration to its short- and long-term aftereffects—remain difficult to pin down, we now have enough data to identify four broad stages of infection that can guide us going forward.

To grasp the complete picture, it helps to look back on how each individual piece of the puzzle fell into place. The story began, like most mysteries, with an emerging problem and a complete lack of knowledge. In the first few weeks of the outbreak, Covid-19 appeared to be nothing more than a serious pneumonia. Symptom-wise, it seemed similar enough to what some people experience when they contract a severe case of the flu.

A month went by. Researchers in China, and soon enough elsewhere, began to realize that the lung damage associated with the disease was more complex than previously thought. This was due in large part to an overreaction of the immune system known as a cytokine storm. Not only was the pathology of autospied lungs atypical for viral-induced pneumonia, but high levels of inflammatory chemokine and cytokines were present as well.

About two months into the pandemic, physicians around the world began noticing hypercoagulation—rampant blood clots, in other words. Heart attacks, ischemic strokes, and lung emboli were noted, as well as the incidence of blood clot-related kidney disease.

More recently, evidence has emerged that suggests the existence of a late hyperinflammatory phase. When Covid-19 patients enter this stage, the lining of the blood vessels—arteries and veins alike—becomes inflamed. Skin rashes, Covid toe, and other markers of hyperinflammation appear.

This raises the possibility that much of the late—and highly lethal, I might add—aspects of the disease don’t occur because the virus is continuing to propagate itself, but because replication-competent sub-genomic fragments persist that are partially sequestered from immune recognition. Such pathology would be unusual, but not unprecedented.

The chart below, created by infectious disease specialist Dr. Daniel Griffin, represents our current knowledge of how Covid-19 progresses, from the moment of infection to no certain end. The pathologies described above can be summed up in four phases: the viral phase, cytokine storm phase, coagulation phase, and late hyperinflammatory phase.

Aspects of the infection not depicted in this chart include damage to insulin-producing islet cells, which leads to diabetes; damage to the splenium of the corpus callosum, which leads to neurological complications; and damage to cells of the liver and heart. More pathologies are to come, I’m sure, as the number of Covid-19 patients increases.

That so many Covid-19 patients go on to develop such a diverse array of adverse health outcomes is troubling, to say the least. But the more we learn about this disease, the more capable we become of developing tools to mitigate it. My hope is that those come to fruition sooner rather than later.