It's around the mid-2300s. Earth is, to put it simply, in trouble. Climate change has resulted in flooding of major coastal cities, displacing billions, which ultimately led to widespread conflict and death. Resources are dwindling, which leads to further conflict between major world powers.
There are colonies off-world. Habitats on the Moon, and cities on Mars, which is in the process of being slowly terraformed. There are populations on Ganymede, Titan and Europa, and orbital cities around Jupiter.
In the midst of all this, from the depths of interstellar space, a small craft of some kind comes hurtling. It's cloaked to satellite detection and it travels for years until it finally lands on Earth. The craft contains a strange extraterrestrial life-form - basically just a small, black mass, pretty much a gross-looking goop.
It lands in, let's say, a small forest somewhere in China. A forager wandering around finds the craft and checks it out of curiosity. Unfortunately for him, the black goop senses life and essentially takes over the man, making its way through his oral cavities and into his brain. This is Patient Zero - but he doesn't die.
In a strange quirk of handwavium biology, the life-form, after burrowing itself into the man's head, produces a highly contagious airborne virus-type thing. Once the virus gets into another person's body, it's supposed to grow into another organism like the initial life-form.
But that's not what happens, at least, not to everyone. The vast majority of people contract the "virus", but it doesn't grow into anything. It incubates within their body, and given that now population density is even greater due to displacement, and air travel is a given, soon, the vast majority of the world is infected.
After a few months, the deaths start, almost all at once. Hundreds, then thousands, and then millions. It's too fast for world militaries to do anything about it. The deaths cause widespread panic, unrest and riots. Martial law is enacted but again, too little too late.
As the death toll gets into the billions, two other things emerge: some people are immune to the virus, as in it doesn't get into their bodies and affect them at all. Furthermore, there are some people for whom the virus did work, and actually grew into the parasitic life form in their heads.
The world lies in ruins, and as the survivors try to make sense of what happened and rebuild, this new threat emerges. The "infected" can "manually" force the parasites into the immune, and grow their numbers that way. The parasite is an intelligent creature; it's actually fleeing its own ravaged homeworld (there's more backstory on that), and its only goal right now is to infect as many as possible and create an army of host bodies.
The colony worlds have no idea what to do. They're too busy trying to survive on their own worlds and create something stable in extreme and hostile environments. They don't have the means to come to Earth and help - Earth is on its own.
Does this make sense? I know it's far from hard science or realistic but just based on its own merits, does it seem plausible? Any holes I may be missing?