Prepare for biological warfare
Simple things might be our greatest defense against aliens, but what about more complex organisms, such as the flu virus? H.G. Wells’ science-fiction classic “War of the Worlds” mentions that flu and bacteria — as "God in his wisdom has put upon the Earth" — could spell the end of invading aliens. So it's worth a shot to sneeze and cough in the direction of alien invaders. Finally, don’t rule out a biological weapon that might appear as a result of an alien/human hybrid. As evidenced in the TV miniseries “V,” a new bacteria based on the alien invaders could be used against them.