That's a great question—because it forces us to look deeper into the story, and learn even more from it.

It doesn't come as surprise that our actions affect those around us, but less common is the understanding that we influence not only the people in our lives, but our surroundings as well. The world at large could not remain upright in the face of the moral decadence during Noah's time, and even the animals were corrupted. The Midrash tells us the animals cohabited with other species not of their own kind.

So when Noah gathered the animals to be saved in the ark, only those which had remained faithful to their own were allowed to enter.