Alien lifeform (something like the blob) deadly to all races ,so dangerous that it is blockaded on a planet. Somehow a ship lands and what is left of the alien escapes to Earth and crashes in the ocean, I think. Aliens send out a ship to try and find it so they can trap it again. Alien comes in contact with a human, turns out it had just been looking for a partner/host one compatible with it. The race of man is and it starts to spread and mankind improves as it links mankind together.
It sounds a bit like Isaac Asimov's Green Patches. But that was a short story and the details don't quite match.
The planet's creatures have green patches instead of eyes, and they are all linked to one organism. They feel badly for the humans who are all separate and want to save them. At one point a very small organism hides on a human spaceship, looking like a piece of wire, and plans to get to earth and "fix" humankind (who sees this as a threat. They don't want to give up their individuality).
The part that doesn't fit is that he doesn't succeed in "improving" mankind and linking them together.
Could this be it?