[RobertG (OP)]

When you hear a healthy young man like Dr Xander Van Tulleken has a bad case of C19, you realise it's not just the old, the fat and unhealthy get it. As a layman I don't have the stats, but it seems that, if say 25% of tests are positive, and  only 10 to 20% of those get it bad, it seems to me  that there must be a special marker in those cases that the majority don't have. It can't just be random. Maybe it's low LDL as reported from China a while ago, or something else. Are the clever research scientists working on this?

[set fair]

A marker may be there for some people ie there is something in their genes or the expression of those genes which makes them vulnerable. Often severe illness involves an overactive or overactivated immune response or innapropriate immune response. Vitamin D helps regulate the immune response and a dericiemcy (which is shockimgly common) may be part of the problem.

I think an upregulation of the response of the nuclius of infected cells may be an early step in serious illness.