It is an NIH piece that hints at the question of progression, equilibrium, etc. From http://science-education.nih.gov/nih...l/biomed1.html

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She and others have shown that the immune system and the brain and the rest of the nervous system "talk" to each other through a number of chemical "mediators" -- cytokines from the immune system and neurotransmitters (like serotonin) and hormones from the nervous system.

The chatter among the mediators is elaborate and complicated, but the goal of all their signalling is a common one -- to keep the body's brain and immune systems in balance (6). When balance is lost and the host encounters an inflammatory trigger or psychological stress, diseases as different as EMS, arthritis, seasonal affective disorder, multiple sclerosis, and chronic fatigue syndrome can develop. It is often difficult to identify which mediator has signalled too much or too little in a sick patient, notes Sternberg, because, by the time the patient seeks medical help, the whole signalling system may already have been thrown out of whack.

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Sternberg says that every person is born with a unique set of genetic predispositions that affect disease susceptibility. Some of these predispositions may be so strong that, even with the tiniest exposure to a disease-causing or inflammation-causing agent, the person will get sick. Others may be much weaker, and, in those cases, whether the individual will get sick or not will depend on other factors. One such factor is the stress response. <END>