The Drake equation (
https://en.wikipedia.org/wiki/Drake_equation) and the Fermi Paradox (
https://en.wikipedia.org/wiki/Fermi_paradox) leave me to believe that only the most peaceful and long-term thinking of species / civilizations would survive long enough to become an extraplanetary civilization and be well on their way to a Type II technology level (
https://en.wikipedia.org/wiki/Kardashev_scale) - otherwise they would have destroyed themselves one way or another. This could mean that any advanced civilizations are stable and content within their own stellar neighborhoods and refrain from contacting us due to either a Prime Directive-like prohibition OR a caution towards broadcasting a signal lest agressive civilizations (including perhaps ours) set their sights on their star systems. They could still be listening to our signals and evaluating us.
Another possibility is that advanced civilizations have transcended tribalism, emotional responses, and perhaps even biological bodies. Imagine a space-faring race of robots that are mearely indifferent to us. They don't care what we are doing on our planet. They arrive at our solar system, mine some asteroids, rebuild / reproduce, and leave. Why would they bother going down into a gravity well?
Regarding a possible weakness of biological aliens to our microbes (bacteria, viruses, etc.), I think the chances of a species developing either a generational ship or relativistic spaceflight without having advanced microbal defenses are incredibly small. Right now, our civilization seems within decades of surpassing the WWII era medical technology (anti-biotics, vaccines, and simple epidemiology) to arrive at a biotech future: rapid gene-sequencing of every new strain of virus; computer prediction of proteins synthesized, cell structures, and weaknesses; and nano-bots in the bloodstream to act as surgeons, pharmacists, and security guards on the cellular level.
The thing about microbes is the details may differ, but there are only so many ways to make a reproducing ball of chemicals (bacteria, amoeba, viruses, prions). Our variety of microbes are extremely likely to have evolved anywhere that ever had primitive life. Therefore aliens should have evolved or technologically developed defenses against these disease to the point that any threat to them would be either manageable or negligible.
In the unlikely scenario of there being a biological species intent on conquering every habitable planet, a lot might depend on their motivations - resources, slaves, or a token of bravery. Again any civilization that can travel interstellar distances must have extremely long-term planning as one of its qualities. Sure if they desire to show off or want us as slaves (at the current 7 billion level), maybe they would try to destroy our militaries and infrastructure and then start abducting us.
But if they just want our resources, what they would probably do is redirect about a few hundred big asteroids to impact our population centers all at the same time. It would take really good calculations, very basic propulsion technology, and a lot of patience (maybe 100 years). The impacts damage to population, infrastructure, the likely "nuclear" winter for the next few years, and fighting among old and new factions would probably reduce Earth's population from 7 billion to 70 million in 20 years. Plus most of the warheads, munitions, and bullets would be used up by then. Its a real easy way to soften up Earth as a target. The best part is that there is almost no chance that with today's technology we would be able to know who did it to us. We probably wouldn't detect them until it was too late. Some scientists might even think it was a big coincidence at first.
