Consider the possibility of simple polymerizing molecules that are not living, but are biologically very active (and harmful). Prions are not chemically very complicated but can be incredibly lethal.
As far as my understanding goes, prions require contact with either the same or extremely similar proteins to have any affect, so they would require even greater biological similarity. Are their prion-like agents that work differently than I suppose?
> prions require contact with either the same or extremely similar proteins to have any affect
There is no requirement for similarity or sameness. Even if there aren't any that I know of that violate this - it's not in the definition that prions require contact with either the same or extremely similar proteins to have any affect.
Contamination doesn't even have to be self-replicating to be a major concern. Many organometallic compounds are exceedingly lethal and accumulate in biotic life. If alien tissue uses any to a significant degree, it would have disastrous consequences.
Obviously, the dangers of runaway self-replication makes even the smallest samples of primitive life potentially more deadly. And, while primitive life is far more likely in the universe at large, it is astronomically unlikely to reach our solar system undirected.
But when talking of other solar system bodies, like Mars, isn't it the other way around? I.e. astronomically unlikely that contaminants haven't already found their way to earth, undirected, for billions of years now?
