Lymphadenopathy (swollen lymph nodes) is a standard symptom of immune activity, e.g. as a response to infection or, in autoimmunity, an autoimmune flare. It is a typical symptom of ME/CFS. It does not mean that any infection is IN the lymphatic system, let alone one that can't be shifted. It might be, but there is no evidence that I know of that says it is. There's quite a good primer on the immune system including the lymphatic system
here.
We appear to have sluggish/blocked lymph ducts, which could be one reason why our lymph nodes are swollen so much of the time. Lymph is normally moved along the lymph ducts by skeletal muscle contraction, so that one can see how lymph flow might become sluggish if someone is physically inactive.
That's the basis for manual lymphatic drainage/massage techniques (e.g. the Perrin technique) which some people find symptomatically beneficial - they purportedly move the lymph along the ducts, which could flush out toxins and reduce swelling.