The scientists are discovering predisposing factors with C-19 as time goes on, as there are with any illness - diabetes, heart disease etc.
In my case of M.E., my predisposing factors were a huge amount of longterm stress ongoing for 10 years (husband an alcoholic, then very messy divorce), I was very run down, caught a lot of infections and had loads of antibiotics. Then 3 house moves in 3 years, caught a nasty virus and wham !!! Was bedridden for 2 years until someone in the local M.E. Link Group recommended a naturopath, and from then on it's been alternative therapies all the way.