They suggest that in CFS, we often get hidden, concomitant parasite, yeast, and viral infections indicating a weakened immune system. One infection puts demand on the immune system which can't kick it, and another infection may join in, leading from one to another in a domino effect. Of course, there is considerable disagreement on the multiple infection theory. When that happens, it is fair game for various viruses and infections to attack the victim. What is quite clear is that CFS is multidimensional.
Several factors contribute to it and, to be successful, it needs to be treated holistically.