Think this "old friends" theory has been going for some time, but find it a fascinating idea that what we thought of as simply parasites could actually have some kind of symbiotic relationship with us (though I suppose it's well known herbivores' caecum's contain bacteria that help their digestion). Anyway, maybe one day will have the benefits without the downsides.
But especially interesting was the following, suggesting some kind of evolution/mutation occurs to accomodate both organisms if placed together.
Bacteria were introduced to a group of amoebae. The amoebae did not like the bacteria and tried to kill them - but could not.
And five years later neither organism could live without the other.
The amoebae had deleted certain genes in their own immune systems and the bacteria had done the same so they could coexist peacefully.
As a result, the amoebae no longer had a complete genome unless the bacteria were present.
Professor Rook said: "It now looks more and more likely that the development of our regulatory immune system depends on molecules that are encoded not in the genome of the human but in the genome of some other organism we lived with throughout history."