Unnatural History Season 1 Episode 2 (Unnatural History s01e02) with title “The Griffin Gang” airs on Sunday, June 20th 2010 at 08:00 pm via Cartoon Network.
Here is summary for Unnatural History Season 1 Episode 2 :

An old germ is released following the discovery of a Pony Express mailbag, forcing Henry to track down a cure in order to protect his fellow students.

Few hours after original series aired on TV, online streaming video for Unnatural History Season 1 Episode 2 usually available on Net and of course, if video available, as soon as possible will show you where you can watch this episode for free.