flu-like illness with exhaustion, severe flu-like muscle aches, and clear cognitive dysfunction within hours.
@SOC - my third infection (2007) was much as you describe here. It lasted longer than any flu, clear symptoms of mild encephalitis in terms of impaired thought processing of a strange sort. I was left with significantly more memory loss, significant impairment of executive function, chronic muscle pain for several years, and progression of neuropathy that had been more smoldering prior to that.
I did not even try to get diagnosed at the time of acute infection because of the brain dysfunction, because my PCP had recently left his practice, and because of too many prior experiences of not being believed. Testing at some point showed elevated titers of coxsacki Bs, adenovirus, and EBV - the probable source of my original illness. Another severe infection a couple years earlier was never tested but doctor said it looked like coxsacki.
Fluctuation in the intensity of the symptoms that became chronic followed an odd pattern that suggests reactivation - muscle pain and/or tendonitis in my legs and cognitive and memory impairment have tended to worsen and improve at the same time. I think I had noticed the same association with edema in my legs and feet that has never been connected to any significant structural heart, lung, or kidney dysfunction that is usually the cause.