It would, indeed, require far more time and energy than I have available to do this justice, but I will make a pass at it. Please indulge me to this extent- Unless noted as "IMHO", I can cite a peer reviewed source for any of this but it would take forever. If a critical point needs it I will try to find it here on the "Hard Disk from Hell" but I ask your indulgence otherwise.

Start in the middle. Causes come from one side by multiple paths. Effects leave in the other direction and quickly branch out to our individual manifestations. In the middle is a system-wide collapse of our ability to maintain balance (homeostasis). Insert "IMHO" there somewhere

So we need to picture an hourglass shape. At the "waist" of the hourglass lies a slow motion wrestling match between your immune system and your endocrine system. At ringside sit your GI system and central nervous system (CNS). They take some damage and add to the confusion, but the main event occurs as your increasingly exhausted endocrine system seeks to control your overly alert immune system.

The struggle can begin at middle age as a result of normal increase in the "touchiness" of the immune cells. This is senior onset. It can begin in your twenties with a challenge such as the flu followed by an incomplete shutdown. It can begin in the womb when your emerging immune system is influenced by maternal immune responses. It can even begin two or three generations back with events which altered your mother's endocrine system and which are reflected in your own make up.
(to be continued - I have a ride coming)