The background is that Earth is the victim of a massive comet strike, that wipes out a large portion of humanity (let's say 80-90%). Let's say that the comet was carrying an alien parasite that's a kind of super-extremophile, managing to survive both the cold void of space as well as the extreme temperatures from the comet impact.
After the impact, with the Earth in ruins, how does this parasite spread among humans? The goal for this parasite is to multiply using humans as hosts. What would be the most effective way for it to find the first host so it can start multiplying? Let's say that by utilizing the host, the organism can multiply and then pass on the offspring organism into other humans.
The issue is that now there are considerably less people and they are spread much further apart. Is it possible that maybe an expedition was sent to the impact site a few years after impact, once the Earth has stabilized a bit? Maybe the expedition is from a large settlement and they went to check to see if they could get raw materials, only to come in contact with the parasite? They would then bring it back to their settlement, where it now has an opportunity to spread.