This is such a weird disease, I cannot believe it is naturally-occuring.
There are evidently different strains, and also, it is speculated that the initial viral load you get can determine how severe you case is. Then, besides the pee-existing conditions aspect, there are the environmental issues, from staying indoors and not getting sun and fresh air and exercise to exposure to chemicals.
I got the flu about the 1st week of Feb, and which I had before with the usual symptoms, that of initial drug-like light-headedness, followed by headache and pronounced fatigue, but no sore throat, or cough and just occasional sniffles, and normal aches. This flu passed but the headache (not a migraine type, more like interior pressure) remained for weeks, and which I self-diagnosed as encephalitis. However, I had good energy and wanted to get out, though it was winter. I finally prayed about it, and the headaches have slowly decreased as I got outside more, though it still feels like there is some swelling inside my head. Spent all day in the garden Thurs and Friday, and after about 5 hours of sleep was out the next day, praise the Lord. I am 68, and about 135 lbs, but I never went to a doctor for this (or any condition since the early 80's) praise the Lord, and no one else in the house came down with what I had, though twice weekly we sat a table and prayed, sang hymns and read the Word. Thanks be to God.