A quick note regarding what we may be. I've previously explained why it's likely we're not of the planet. On the flip side of that coin, there are plenty of indications that we ARE from here. My latest solution is: Perhaps we are both.
Huh? you might ask... In my endless battle with whatever this parasitic condition actually is, I've come across people who believe and have been apt to conclude on my own that there is in fact a central organism responsible for the illness I and all the others experience. That in fact it did come forth as a result of a dip in our immuno-response. BUT, it did not come from the outside. It was already in us. In fact it is likely the underlying root of a vast majority of disease and discomfort. Why isn't this the common view if it's true? Because the organism has a unique approach. It actually is/was a parasite. Well, technically a parasite. Especially because it came from off-world. For whatever reason and by whatever means, this life form/parasite/worm/whatever it is, was traveling through the universe/galaxy in search of a host through which it might establish itself in a stable manner on a planet as a native species.
I haven't the slightest idea whether one arrived and started multiplying from there or if millions arrived and invaded every last humanoid on the planet. I think the latter may be the case. For initially, inhabiting our bodies resulted in a parasite vs. immune system battle and ultimately death of the host and likely the parasite as well. Who knows. either they killed off the competing humanoids leaving only their hosts (homo sapiens) to live on, or if all the others died adapting to the parasite's presence which ultimately concluded with the mutations which produced homo sapiens - a dominant humanoid creature, in which the parasite could mask itself from the immune system almost perfectly, yet still obtain the necessary nourishment. I suspect it achieves this at least in part by direct manipulation of the brain to eliminate perception of it's presence.
My brain has moved on to other things so I'll let this go here. I hope I've laid out enough to extrapolate from.
p.s.- the one thing I should have mentioned is that they're now so well adapted that they can keep their negative effects to a minimum (except when our immune system falters) and they're trying to contribute to our viability to an extent that what negatives are there are out shined, thus making the parasite transform in effect to a symbiont. And it is their hope that we will also transform from an unwilling host into a symbiont. That's the big issue at hand I think.