So I don't remember the plot line exactly, but the dream was very vivid and strange. Anyways, general points:

Aliens were trying to take over the world and were possessing humans through the base of the spine.
The possessed humans could be hurt by punching them in the right side Where they resided.
The possessed humans would say 3 words, pause look around, and then continue. It was a very jerky way of speaking.
The aliens didn't know the differentiations between males and females and often times the females and males would be thing that would be out of character for them. Example being a woman standing at a urinal and then leaving.
There were small groups of people that figured out the patterns and were able to blend in for the most part.
The main alien possessed John Lithgow (yes he had a starring role in my dream). Lithgow's character had cancer of something on his right side, and so the punching in the right side didn't work so well...
It was semi-serious and yet a parody at the same time...