One thing interstellar colonization has going for it over planetary scale is that the plausible returns from an interaction with an alien civilization are by necessity not the same kind of resource grabbing nonsense we saw on earth. It is wildly idiotic to waste so much time, energy, and money stealing raw resources out from under the little green men, as opposed to mining in our own system in an inhospitable environment. it's also laughably unlikely that we will be able to infect one another with anything, or even use each other as food. If life on earth was partially or wholly seeded from outside sources, it becomes more plausible that we could be a little bit like whatever we meet out there, at least on the molecular level, but even then it's rare enough for a disease to jump between two different mammals, the differences between planets would be enormous.
Personally, I do think that dropping bacteria bombs on planets with small scale existing ecosystems would be a catastrophe. I think that interacting with intelligent life, should we ever be lucky enough to stumble upon it, is completely inevitable and as such it's more important to focus on getting that interaction right than deciding whether or not we should. It only takes one person to open their mouth, after all, and an entire planet to keep quiet.
One thing that not a lot of people realize is that carbon is very likely the only molecule that can serve as a basis for life, because of its unique bond number. So, if we encounter other life forms, it is very likely that they will also be carbon-based. A lot of what forms "life" is simply reliant on universal properties of atoms.
In short, we will probably be able to eat them, and vise versa.
That high bond number also allows for a lot of stuff that's viable but isn't actually used or usable by earthly life. One definitely viable and definitely not usable by earth creatures (or at least not by mammals, for all we know there could be some undiscovered microorganism or fungus or weird sea creature that can make use of them) is chemicals of opposite chirality.
As for cross-infection by pathogens, viruses and plasmids would likely have a very difficult time given that the the aliens genes, even if based on similar chemicals, are unlikely to use the same base pairs (there are plenty of perfectly viable nucleobases and chemical relatives thereof that aren't used in DNA (some of them, though by no means all that are possible, are used in RNA. Uracil is of course the most famous, but you also occasionally get things like inosine/hypoxanthine). Furthermore, even if they used the same bases, it is unlikely that the same combinations would code for the same things; The genetic code by which dna codons of earthly organisms are translated into proteins is highly conserved but variations can and do exist (mitochondrial DNA is translated slightly differently than the regular dna of the rest of the cell). They might not even use the same amino acids as us; there are plenty of amino acids out there with no corresponding codon.