Polycythemia rubra vera is a bone marrow problem in which the marrow produces too many red cells. It is not exactly cancer but has a lot in common with cancer. Normally a chemical signal keeps the marrow from producing more cells when they are not needed. But for approximately one person in 200,000 the cell DNA begins to change so that these new cells no longer recognize that signal. Onset is usually in the thirties or forties, I'm told. Mine became noticable in my mid-fifties and I've heard of onsets much earlier.
The most common symptoms are a flushed complexion, cold sweats and itching of the trunk of the body, back, and/or uppr arms or legs. This is sometimes very severe and often leads to sleep deprivation. Left untreated, death would follow in a period of months or years. There is no cure but with proper treatment and a little good luck for a change life expectancy can be extended to twenty years after diagnosis, sometimes more.
Most commonly, treatment consists of periodic phlebotomies, which is the polite term for blood letting, but there are also meds that can be substituted. As my doc puts it, "You are more likely to take this condition to your grave than to have it send you there."
Be of good cheer. There are a lot worse things lurking in the dark of the world.