I know there's a guy, or maybe he represents a group of researchers, who thinks that Chronic Fatigue Syndrome is caused by inflammation in the brain, either due to some undetectable infection, or just a misfiring that is a lingering effect of some other insult (sounds to me like a histamine response aka allergy).
If your body thought you had the flu but you didn't, you'd be knocked on your ass not unlike many people with CFS or Lyme. And if it never stopped you'd be in pretty bad shape a year in.
So. Are we treating microbes that don't culture in a petri dish, or are we solving an autoimmune response gone whacky? You could really ask the same question about Crohn's or IBS.
I recall years ago reading a long story about a consultant who was called in to figure out why this black soot was all over a neighborhood around a brewery. The brewery said it was diesel exhaust. But why only this neighborhood? He heard of a similar problem is some town in the UK. But Petri dish after Petri dish cultured nothing that could explain the black soot. Ergo, it's not microbial.
Our intrepid consultant pours a small amount of distilled spirits onto a sample dish, and gets a bunch of black shit. The 'soot' was feeding off the Angel's Share (evaporative losses from the brewery), and would barely grow without it. Vigorous growth in vivo, negligible growth in vitro.
We've come up with a cure or treatment for so many microbes that we can detect, but how do we know we're detecting them all? People die of cancer not because cancer is getting worse (well, pollution notwithstanding) but because Cancer is patient. There's a lot of space in the middle for new vectors to slip in.