It all started with prosthetics.
Once a person could completely replace an arm or leg (and eventually any organ) with a cybernetic lookalike that was in all ways superior to their original body part, people clamored for new and better ways to replace themselves.
We aren't talking bulky metallic machinery, but sleek nanotechnology that can mimic flesh, while having the strength of a superhuman. That was just the start, of course, because as soon as you could replace one limb, why not add another? And another? Why not have three or four or ten arms?
Heck, why not have remote arms? It was one additional step to remote bodies, and one more step to the brains in vats stage, where everyone controlled bits of things removed from themselves, while encased in their life support system.
What was the point of even having biological arms and limbs and digestive tracks and lungs and eyes and skulls? After all, eating and moving around and breathing became expensive with 100 trillion people crammed onto planet Earth. It was much more cost effective for everyone to live as brain and spines encased in giant neural networks.
The virtual melded with the real, as awareness shifted to peripheral perception devices. A "person" could "see" with computer video cameras, hear with microphones, and touch with robotic arms that could be anywhere in the world. Not to be outdone by natural selection, humans could also sense every other frequency with additional nanocomputerized instruments.
Banks upon banks of humans, stacked a billion to a square mile, neatly organized in rows and tended by automatic drones, thought and thought and thought, a mind of minds.
When Earth became metasentient, it decided to procreate. It made Mars and Venus and everything in the nearby solar system in its image. Now the Earth was a metaspecies. Its cells were people, and it metathought with a metabrain of brain-neurons.
Not to be limited to one solar system, Earth learned space travel, and dominated nearby systems. Each planet became a new host, filled with trillions of brain vats, and each new Earth could metathink, and metafeel. They could go on metadates, buy each other metadrinks, and eventually, if they metaliked each other, they could have metasex and make little Earth babies.
Though it evolved to be something unlike the original Earth (just as simple cells evolved to complex cells over 1.8 billion years) the metaspecies prospered. Having spread to the farthest reaches of the Milky Way, Earth was not content, and so decided to dominate nearby galaxies, so that Earth might spread to everywhere in the known universe.
It was at that point that the metaalien known as Universe-348 went to the doctor and got a prescription for antibiotics specifically designed to fight off an Earth infection.