It was an old radio broadcast that caused most of the population to freak out because they missed the disclaimer at the beginning which said that the story was fictitious, so when they tuned in they thought there was an actual alien invasion.
Anyway, from what I remember, aliens come down and kidnap most of the population, only a few survivors left. After about 24 hours, all the aliens are dead because they had no immunity to earth diseases. This is based on real world stuff. Eg, when the Spaniards discovered the Americas, a lot of the natives died off from the European diseases which they had no immunity towards.