I think it's everything follows the same basic patterns.  Like the shape of an atom with its electrons circling around the nucleus, the solar system with its planets circling around the sun, galaxies with the stars/solar systems circling around the black hole at the galactic center.  Life is the same.  Virus infect lifeforms on planets, "life" infects planets in every galaxy.  I would bet there are far more inhabited planets than the Drake equation assumes.