This is one of the things that I like about the expanse series. When people first start moving out to alien ecosystems, they specifically go into how alien viruses etc are probably not going to be a problem. The metaphor they use is "mining" basically. Where they're not concerned that people will get infected, but they're concerned that some organism will find out that people are full of some resource that the organism can use.

And in fact they bump into some sort of algae that lives in the atmosphere that really likes the saline mixture that makes up most of the liquid in our bodies.