Supernatural Season 6 Episode 5 (Supernatural s06e05) with title “Live Free or Twi-hard” aired on Friday, October 22nd 2010 at 09:00 pm via CW Network.
Here is summary for Supernatural Season 6 Episode 5 :

After a group of young girls go missing, Sam and Dean realize that a group of vampires have taken up residence in the small town. After a fight with two of the blood-suckers, Dean is bitten and turns into a vampire. Samuel tells Dean he can change him back but he’d have to get the blood of the vampire who turned him. Dean infiltrates the nest and runs into the Alpha.