The New Scientist has this coverage of evidence that a small parasitic fly Apocephalus borealis may play a significant role in Colony Collapse Disorder in North America.  The parasitic fly causes the bee, in this case, to become disorientated and this in turn helps spread the parasite to other bees in the area.  Full report.

This reminded me of the effect Toxoplasma gondii has on the brains of rats which also has the effect of allowing the parasite to spread more rapidly.