Curious - do folks in this prodromal phase of leukemia / lymphoma experience post exertional malaise?
Most such evidence is anecdotal, like the report from Dr. Michael Snyderman, an oncologist/hematologist who was diagnosed with "CFS" a year before he showed clinical evidence of chronic lymphocytic leukemia. This is by no means an isolated example; it is noteworthy because the patient is also a specialist in treating the disease.
(His case became involved in the controversy over XMRV. He has been experimenting on himself with antiretroviral drugs, and his tests do show the "numbers" changing in response to changes in medication, though this does not tell us which retroviruses, including HERVS, might be responsible. He remains alive, longer than expected without conventional treatment for CLL. The fact that he needs to periodically change medication to control disease is no different from patients with known HIV infection.)
If we had more research on PEM we might be able to say. Severe fatigue, which gets worse after exercise, is typical, but current research on PEM has only found the characteristic prolonged drop in VO2 max and anaerobic threshold in ME/CFS patients. Nobody has even been looking for this in other patients, and while
some MS patients report something similar I don't see how research requiring MS patients to exercise to exhaustion could pass ethics review. This is a classic Catch-22 situation: if you have either MS or leukemia/lymphoma you can't be tested for the known distinctive characteristics of PEM. If we had better research criteria we might be able to detect the biochemical changes without risking patient relapse. You can find lots of work based on psychological hypotheses and measurement by questionnaires. You can even find some such in MS and leukemia/lymphoma. What these results mean is questionable.
Added: the possibility that a single cause could result in such different diseases should not be too surprising. HTLV-1 is known to cause both a neurodegenerative disease (HAM/TSP) and adult T-cell leukemia (ATL). We have also learned that the majority of those infected are asymptomatic for up to 40 years. Properly functioning immune systems can cope with the infection.