“The body maintains its vitality not only through the physical motions and agility that you perceive, but by microscopic agility, and actions within microseconds, that you do not perceive. There is as much motion, stimulation, and reaction in the interior bodily environment as the body meets through its encounters with the exterior environment. The body must now and then “flush its systems out”, run through its repertoire, raise its temperature, activate its hormonal actions more strongly. In such ways it keeps its system of immunities clear. That system operates always. To some extent, it is a way that the body distinguishes between self and nonself.
“In certain fashions, that system also keeps the body from squandering its energies, preserving biological integrity. Otherwise it would be as if you did not know where your own house began or ended, and so tried to heat the entire neighborhood. So, some indispositions “caused by viruses” are accepted by the body as welcome triggers, to clean out that system ...”
(Dreams, “Evolution” and Value Fulfillment, Session 906)