So here's a question. Let's say we find a planet that has its own carbon-based life. Even if it was within the habitable range for humans, could humans ever live on that planet? Or would the threat of microbial nasties that we had no immunity to make that possible?
That depends. We probably wouldn't be in danger from their viruses, since they might have a different genetic medium than DNA, or at least one following a different protein-coding "language." But bacteria could be a problem if our biochemistry is compatible enough with theirs that they could thrive in our bodies. And that's pretty much a given if we could thrive in the planet's environment. The safer we are from a planet's bacteria, the less suitable the planet would be for us to inhabit at all.