Actually, the real risk is that, by entering in contact with Earth biology, some forms of life incorporate them and develop in a toxic way.

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.