It will be like a prion disease such as (https://www.cdc.gov/prions/index.html), but utilized in a way that's similar to CRISPR gene editing (https://www.technologyreview.com/s/609722/crispr-in-2018-coming-to-a-human-near-you/) except it'll rain from the sky during a meteor shower from the debris of an asteroid with a hyperbolic trajectory (https://www.space.com/38580-interstellar-object-spotted-comet-asteroid-mystery.html) or the worse case scenario is that it directly hits Earth and wipes us all out except for the prions to start this mess all over again. Even we earthlings can land probes on comets (http://www.esa.int/Our_Activities/Space_Science/Rosetta). Think of it as the alien equivalent of our golden records on Voyager 1 and 2 (https://voyager.jpl.nasa.gov/golden-record/) but with genetic information that says "this is what we are" by species mutation. However, the odds of us being genetically similar enough to be susceptible to their prion is hopefully unlikely. I've often wondered if Mad Cow and similar diseases are just an alien Voyager failure. We may have come close; notice how the cow is number 9 on this list (https://www.thedodo.com/animals-you-had-no-idea-were-so-closely-related-to-humans-1172946617.html). But for now, we can just monitor the fruit flies (https://science.nasa.gov/science-news/science-at-nasa/2004/03feb_fruitfly) as an inadvertent, invasion "litmus paper." A prion "disease" has a better chance of surviving a long space journey than even the robotic invasion theory does. Besides, we humans are a base-10 species (https://www.thoughtco.com/definition-of-base-10-2312365), so they'll have have to be close enough and long enough to learn our mathematics (1-10, symbols, and axioms) to hack our systems anyway. And if AI develops further, it'll act as a temporary firewall and hopefully log enough to know flag an alert. At any rate, if it is robotic, and they figure and Google or Facebook, they'll know to attack Windows first followed by about 2.2 billion people on their shit-list. But, my money is on "random chance prion" because I doubt they use anything close to our file formats. The only reason we can "crack" any of them is because we know what patterns to look for, most of which would be uniquely human. The risk of long journey mechanical failure is too high anyway; it would be better and cheaper to drop genetic material on passing asteroids and let random chance do the work.
