Isn't that only a problem if you're completely planet-bound? If you want to play it safe, you shouldn't put all of your eggs into a single basket anyway.
Correct, but while exploring the universe, doing it undetected is actually a great strategy. Imagine self replicating nano bots that embed themselves into asteroids. Everything thinks its just big rocks flying in space that sometimes smash into planets. But its a strategy to slowly colonize the galaxy and put a mark everywhere. Kind of how plants spread their pollen into the wind or have bees and butterflies do their dirty work.