Earlier quoted context omitted.
Do you remember what happened when the europeans travelled around the world? 90% of populations died from new illnesses. They basically saw nothing ill in settling and displacing the remaining population.
Well, alien illness probably won't kill us. Without a common ancestry, it's highly unlikely that anything that uses an alien as a host can live in our bodies.
Further, any microbe hitching a ride aboard an alien may well find Earth's general environment to be easy to thrive in. Think Kudzu, but at the microbial level. Eventually, the alien microbe and/or the natural Earth microbes will mutate in order to out-survive the other.