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.
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.
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.