Did you watch the show / read books or just summary by chance?
In the show those aliens they cannot send anything physical to harm you because they are too far away. Everything they use is just to trick your eyes and mind. They can’t actually hurt anyone that’s why they use humans. And even their device they designed took them millions of years to reach earth.
if you noticed during show they are too busy surviving their harsh planet to deal with certain things. They explain how humans are evolving at faster rate than they did. Why waste time on creating a virus that humans might be immune to in the 10 million years it will take to send it to earth
The concept of virus may be foreign to them. We can’t assume what is common on earth is common to aliens.
And third, even if they could send a virus, how would they do that when they don’t have access to humans dna to experiment on? Again they are too far away.
And last with a virus is you risk the virus evolving. What may be harming humans can mutate to harm things you don’t want it to.
I won’t say anything else without spoiling the books or show because it will blow your mind what aliens are actually capable…and not capable of. If I were you I’d read the books or watch a YouTube summmary of the series if you don’t want to wait for Netflix to finish the series. It will blow your mind