This is something I wrote about ten years ago:
Germs that kill you aren’t usually very successful.
They get a lot of attention. They motivate people to avoid them. Pretty sure you’ve heard of Ebola.
Over time they also kill off all the susceptible people and leave the ones who are immune. Burn themselves out.
Really successful microbes don’t kill you. You don’t even know they’re there.
They're insidious.
Either they're imperceptible, or intermittent, or the symptoms are variable.
So you don't ever notice that you're infected.
For that reason, some of them are now ubiquitous.
Everyone has them. Everyone has them all over.
There are many organisms that live within us and are known to cause long term complications. Many of them are also found in people with those chronic illnesses. The problem is- the correlation between them is often not at a statistically significant level. Since there are often many ways to get the same effect, most medical studies are flawed.
The statistical requirements to prove validity assume a minimum threshold of relevance.
Experiments generally assume a one to one relationship between factors. Unless they are specifically designed for an interaction- they do not detect multiple causes. If x or y cause z, and you're only testing for x, well you're half right, but your results will not register statistically.
Similarly if x and y together cause z, your results will show no correlation.
Conversely, this is also true if your illness has multiple or variable effects. If you are only looking for a subset of the possibilities, your experiment will not show a relationship.
Likewise, they do not account for less than threshold activity. If your illness is extremely mild or intermittent, it will never be statistically relevant in a study. They just won't even see it.
Update:
PG fits all these criteria. It masks itself by altering the immune response and reducing pain- making people think it's harmless. It combines with other microbes to create many different effects. It alternates between acute and chronic symptoms...that is why it is everywhere.
It is also why my brain sounded like a casino when I read the PG review article. It rang all my bells at once. At 2am…