First thing I thought of. It's quite a wild little segment. A worker ant has to carry the infected ant away from the rest of the colony. Eventually the infected ant dies, and this alien looking fungus sprouts of of his head. The fungus then releases a bunch of spores (meant to infect other ants) so it's a good thing he was dragged away from the colony! The whole Planet Earth series is absolutely amazing, do yourself a favor and watch them all if you haven't.