Danger for earth. When we introduce a species to a new environment, sometimes it dies, and sometimes it becomes an invasive species and kills a lot of other stuff. And some of those invasive species are big (like snakehead fish) and some are small (like the smallpox virus or, frankly, SARS-Cov-2). Many experts think there's a risk that an extraterrestrial microorganism could devastate some or all Earth life, given that no Earth life has had a chance to evolve defenses against it.