People who develop severe COVID-19 symptoms follow a fairly regular pattern.
On day one, patients often run a fever and experience muscle pain, fatigue, and a dry or unproductive cough. By day five, breathing is labored, and by day seven, they may be hospitalized. Day eight is when the situation can turn dire: Fluid starts filling up the lungs and blocking oxygen flow — a condition called acute respiratory distress syndrome. That fluid shows up as a telltale "ground glass" look on X-ray scans of the lungs (above).
The pattern of critical cases is alarming to clinicians, and something they're still trying to grasp: It's not just people with apparent risk factors like smoking and chronic illnesses who get severely ill — it's also seemingly healthy and young people.
The virus may replicate quickly enough to trigger the immune system very suddenly instead of gradually, causing it to go "berserk," according to one virologist interviewed by The Washington Post. In addition to damage caused by the virus, inflammation may further open up lung capillaries and cause them to leak more — causing fluid to quickly build up in the lungs, cut oxygen flow, and strain most organs in the body, including the heart (which must work harder).
Still, the disease has existed for less than three months and knowledge is evolving about what, exactly, makes it such a threat to a significant portion of patients.
Why it matters: Understanding how the coronavirus does so much harm could lead to more effective treatments in hospitals and make for promising drug targets.