  Chronic Fatigue Syndrome is a protracted illness condition (lasting even
years) appearing with strong flu symptoms and systemic defiances by the immune
system. Here, by means of statistical mechanics techniques, we study the most
widely accepted picture for its genesis, namely a persistent acute
mononucleosis infection, and we show how such infection may drive the immune
system toward an out-of-equilibrium metastable state displaying chronic
activation of both humoral and cellular responses (a state of full inflammation
without a direct "causes-effect" reason). By exploiting a bridge with a neural
scenario, we mirror killer lymphocytes $T_K$ and $B$ cells to neurons and
helper lymphocytes $T_{H_1},T_{H_2}$ to synapses, hence showing that the immune
system may experience the Pavlov conditional reflex phenomenon: if the
exposition to a stimulus (EBV antigens) lasts for too long, strong internal
correlations among $B,T_K,T_H$ may develop ultimately resulting in a persistent
activation even though the stimulus itself is removed. These outcomes are
corroborated by several experimental findings.
