Day 379 of the alien invasion that has taken over Earth. These are the last two specimens of the human race, which comes to an inevitable end.
Maybe we could burn it out
Then came the partying. The aliens were elated that they took over the planet and had no more worries considering that they are omnivores and could eat anything.
But they were not safe from one thing:karma. 3 months after their grand party some of their compatriots starting experiencing death-inducing symptoms. They were coughing up organs, foaming out of their mouth, exploding into pieces.
Many more of them died off with this virus, and so they decided to burn the dead, but it still stuck around. Their population started declining and they started getting worried, at this point 90% were affected.
But then, some of the young miraculously never got affected. Due to this they started living longer and reproducing more. Leading to offspring with resistance to it. Why? Well it was a mutation to their dna
And so with the heterozygous advantage of the favorable mutation, they overcame the disease and lived happily ever after.