Just a random thought.
While looking at the stars tonight this came to mind.
If we were ever to encounter complex alien life does it not seem obvious that we would poison each other?
Given that we have evolve gradually or in spurts in order to improve our chance of generating future generations. Is it not natural to believe that we also evolve such that we are more resistant to threat by acquiring resistance and defense to threat? Such defenses would include those of a toxic nature. Since some of those defenses would also include a toxic component. For example we as humans have an immune system that aggressively attacks threat, thus toxic to the threat. Now our personal defenses would have evolved as a direct reaction to the threats presented to us over the eons. Thus the complexity of our defenses would both be quite elaborate and in it self aggressive. But at the same time would be a counter point to the other biology around us.
Now given an equally complex life form(s) had evolved in a completely alien environment. It equally is likely that these life forms would have evolved in a similar fashion. However most likely taking several evolutionary branches that our world did not or deemed evolutionary dead ends.
If those two complex biologies were to mix does it not seem likely that we would kill each other by simply coming into contact? Of course there would have to be sufficient amounts of each biology to result mutual death. An alien microbe has less of a chance of killing us in this manor as compared to a complex life form of such mass to be anywhere from mite size to elephant size. ( To pick two easy to relate to volumes. ) As the single microbe would have very little success against a comparative mountain of domestic biology attaching and consuming it.
To me this seems obvious at the moment.
If this proposition is true, then does it not seem likely the reason that we have not been contacted by alien life is that simply because our world is a big ball of plague? Thus why bother with taking to the sentient bags of salt water fresh from the primordial stew!