An amazing clip from Sir David Attenborough's Planet Earth:
These bullet ants are showing worrying symptoms. Spores from parasitic fungus called cordyceps [wiki] have infiltrated their bodies and their minds. Its infected brain directs this ants upwards. And utterly disorientated it grips a stem with its mandible. Those afflicted are discovered by the workers and are quickly taken away and dumped far away from the colony.
It seems extreme but this is the reason why: like something out of science fiction, a fruity body of the cordyceps erupts from the ants head ...
Fascinating (in a very creepy way!). Hit play or go to Link [YouTube]
See also: Six Horrifying Parasites