CDC definition: Clinically evaluated, unexplained chronic fatigue greater than 6 months' duration associated with a significant reduction in occupational, educational, social, or personal activities PLUS four or more of the following symptoms: Impaired memory or concentration, sore throat, tender cervical or axillary lymph nodes, muscle pain, multi joint pain, new headaches, unrefreshing sleep, post exertion malaise.
Who knows. Post parvovirus CFS is associated with preexisting increased stress (PubMed).
Giardia is associated with a fatigue state that fades over 6 years (PubMed). That is one of many infections, I suspect many infections can lead to a fatigue state that was set off by the infection but not due to ongoing infection. Q fever and EBVcan do it as well. Unfortunately treating the infection does nothing as it is long gone.
After culture proven Lyme, fatigue is uncommon, perhaps in 3% of patients (PubMed)
There is none or are none.
At least check out the issue of chronic carbon monoxide toxicity due to faulty heaters or increased insulation.
I remain uncertain what causes CFS; a subset appears to clinically have an infectious onset, if it is truly infectious or, more likely, post infectious I cannot say.