They re immune to Earth s bacteria

Alien visitors to Earth are occasionally depicted in science fiction as being brought down by their own alien nature. Lacking immunity to Earth based bacteria, they all die of infections. This wouldn t really happen. Alien life forms wouldn t come here only to be done in by our bacteria, unless they were related biochemically to humans. Bacteria would have to be able to interact with their biochemistry to be dangerous, and their ability to do that is far from a sure thing.