From the low German geck, meaning “fool” the term GEEK was borrowed from carnival midways and has been transformed into a rather pleasant description of a computer whiz or a studious individual with their nose always buried in a text book. A far different description than that of the  geek found in the back end of some carnival and circus midways in the early through the mid 20th. century.
The saddest life form on the midway was the human "Geek". The billed performer's act consisted of a geek, who stood, usually in the center of a "pit show" to chase live chickens or handle snakes. It ended with the performer biting one the chickens or snakes heads off. 
Often filthy, and dressed in torn rags or jungle costumes, Geeks were sometimes alcoholics or drug addicts and could be paid with liquor, ( especially during Prohibition ) or with narcotics. Many times they were simply mentally retarded individuals who were gladly surrendered to the carnival as it came through town by uncaring family members who were happy to be rid of them. They were abused by the ignorant public who often spit on them or flipped lit cigarettes at them if someone wasn't posted at all times to protect them.
The geek shows were sometimes used as openers for other freak show attractions. They were ostrasized in many communities and most of the big shows refused to even carry them on their midway. It was a matter of pride among circus and carnival professionals not to have traveled with a show that included a geek. The last of the legitimate geek shows faded away by the early 1970's. By then, they were mostly routed in the deep south on small rag bag shows. 
The film noir classic Nightmare Alley (1947) features a sideshow performer (Tyrone Power) who winds up as an alcoholic geek, biting the heads off chickens.Another ,more recent  film is "She Freak" which tells the fate of a woman who eventually turns into a Geek ... (Ok...so I gave away the predicitable ending.) The movie is B Grade but it does contain some great color film footage of the old West Coast Shows back in the 60's and because of the low budget, many of the people associated with that show were used as extras.
There are no geek shows that travel with any carnival or circus today but there are individuals out there who do perform these acts today as specailized theater attractions for their shock value.