I doubt we'll find a "new Earth" any time soon, but it won't be because of deadly micro-organisms. The thing about viruses and bacteria is that they have to match pretty closely, to be a problem. It's like software viruses - if you're running some whole different system, they don't even know you're there, basically. We're swarming with bacteria and viruses, hardly any of which are not characterised and most of which we don't even have to react to. Viruses pretty much have to be a chip off the old block. They can't recruit cellular machinery unless they 'know' what that machinery is.
The bigger problem in regard to microbiology is getting enough of it, not too much of it. We ignore the sea of micro-organisms that thrive in us and penetrate every millimeter of everything around us, all the way down into unexplored depths of the earth's crust, and only pay any attention to the very tiny number that we know, because they're pathogenic, or useful.
We tend to regard ourselves as autonomous machines, needing only air and breakfast to survive, but we're so intimately part of our world that we take it all for granted, and don't see how precisely we are part of our world, our gravity, our day length, our radiation, our atmosphere, our microbiota.
Humans going to another planet would just 'fail to thrive', becoming sickly and weak and sterile after a few generations. If we ever wish to live on another planet, we should first send our bacteria, and give them a few hundred million years to get things set up for us.