I dunno about you, but I would be really, really excited if other planets became infested with Earth organisms. I mean, that's kinda the whole reason for space travel in the first place!
I understand the argument for sterilized launches, etc. etc. but I can't help but wonder if it's holding terraforming back in the long run.
1. If there is life elsewhere, we want to make sure terrestrial lifeforms don't drive it to extinction before we get to see it. 2. We want to make sure that if life is found elsewhere that it isn't due to our own contamination of the site.
The closest equivalent is inter-species infections, like the Avian or the Porcine flus. Having humans near unhealthy ducks or pigs constantly means mutated strains have a higher chance of carrying dangerous behaviour from one species to another.
There are a lot more to say around bio-chemistry basins, carbon-based vs silicon-based (or another compound) — but the idea is not exclusively to protect life there.
http://reducing-suffering.org/applied-welfare-biology-wild-a...
https://foundational-research.org/the-importance-of-wild-ani...
http://reducing-suffering.org/will-space-colonization-multip...
What is almost always ignored is that these animals would have feelings, and many would suffer enormously as a result of being created into Darwinian ecosystems in which predation, disease, and premature death are endemic. Theologians ask why a good God would create so much cruelty in nature. Future humans should likewise ponder whether it's morally acceptable to create new ecosystems containing vast amounts of suffering, or whether they should pause and seek more compassionate alternatives if they embark upon space colonization.
Viruses and bacteria don't have feelings and the vast majority of the biomass on Earth is not even capable of suffering.
A genetically modified lichen, or fungus, that was designed as a stepping stone towards a better Martian biome, and a launch setup to seed them effectively isn't too far outside the bounds of present day work with genetics, and present day launch & deployment systems.
It's far less sexy, but a multi-generation approach to terraforming could yield a usable Mars without too much magic...