Well anyone with an ounce of scientific knowledge would know that any civilization with interstellar space travel abilities would encounter very little trouble in overcoming Earth defenses. Therefor organizing the people to defend the planet would be an exercise in total futility.
Any alien civilization that encountered Earth would IMMEDIATELY QUARENTINE the whole planet; they would be smart enough to recognize mental illness when they saw it!
Any civilization with intersteller space travel capabilities would be very unlikely to need anything that Earth has to offer.
Well, except maybe for vibranium, which is pretty rare.
And magic unicorns for power generation with zero environmental impacts.
We might have a problem after all. But the probability is still that they would avoid Earth as a disease reservoir, as they would have seen broadcasts of War of the Worlds long before they got here.
Of course, if aliens were attracted by massive ego pulses and are coming to worship Barack Obama, they might override their safety protocols and come on in.