Scientists are continually challenged by unidentified strains of bacteria with mysterious origins. Could some of our most crippling plagues and epidemics be traced to the darkest voids of space--or even extraterrestrial intervention? During the Black Death of the Middle Ages, people reported bronze flying ships emitting a strange mist. As recently as 2011, NASA scientist Richard Hoover published evidence of life in meteorites. Is it possible that ancient aliens--as small as microbes--have shaped human history?