habitable planets

[Anomalocaris]

Yes, but those new continents are still habitable.

There is a great range between “some people with great effort can live on it” one the one hand, and “not even the unprepared is likely to die on it” on the other.

You think of the estimated >400 billion planets in the Milky Way, not one will have conditions falling in that gap?

We can't really know, now can we? I guess if humans develop the technology to reach them, they'll probably have the technology to survive on them.

However if diseases here on earth can wipe us out with such success, that other worldly ones would have a 100% kill rate, as we would have 0 immunity. Of course anything about habitable worlds is just 100% speculation.  We have 0 idea about how common life is in the universe.

We have considerability more than zero ideas.  We have a substantial reservoir of knowledge of chemistry and biology that allow us to make informed inferences.

If alien pathogens are evolved, then those Pathogens are just as constrained as we are by evolution to be as efficient in doing what it needs to do as possible by wasting as little as possible on unneeded capabilities.  We had no need to defend against them, hence we likely have zero immunity to them.  They had no need to infect us, hence they likely have zero ability to infect us.

We evolved in different biochemical worlds.  We will remain in different biochemical worlds for a long time after we come into contact.