It is suggested that CFS may work with our immune system. There are two parts to our immune system. One is called TH2 or humoral immunity. This immunity system produces hundreds of white blood cells such as macrophages that attack disease in the blood. The big guns of our immune system is TH1 or cellular immunity which has “killer” cells. This immune system attacks disease in the cells. It is suggested that CFS is able to create a shift in these two immunity systems increasing humoral response and decreasing cellular response. In order for immune cells to fight disease they must be activated or taught to recognize what to destroy. It is believed that this illness is able to invade the young immune cells before they are activated against the disease and prevent them from becoming fully activated. Because immune response to cellular disease is weakened by CFS the disease is able to “hide” in these immature cells. When we are sick our body may respond to illness (or other stressors) by producing thousands of immature immune cells. It would seem then that according to this theory this provides thousands of hiding places for the illness. One defining symptom of this disease is the fact that it "seems" to develop after a severe illness. Ack ,hope I got my facts right, I’m no researcher and my brain isn’t what it should be to contemplate such things. Anyway that’s the theory.