Some of the best scientists available are working on the role of pesticides on immunity and the effect of nosema. As yet, there is no strong evidence that these are the major players in colony decline. 

Highlights from the Journal of Invertebrate Pathology February 2012

> Infectious diseases have not been sufficient to fully explain the symptoms and incidence of colony collapse disorder, and for this reason, speculations have been made regarding the possibility that sublethal pesticide exposures might make bees more susceptible to a variety of common infectious diseases. Unfortunately, little evidence to date exists to test this hypothesis, but it has served as the primary reason why we put together this review.  -- R.R. James, J. Xu / Journal of Invertebrate Pathology 109 (2012) 175–182

> We observed very little impact of Nosema infections on either colony growth or productivity, suggesting that even though we found higher levels of infection, treatment may not be necessary. The low levels of infection during the fall and winter also indicate that the use of fall treatments is unnecessary. Treating in early March may be beneficial for weak colonies with high levels of N. ceranae, but are otherwise unnecessary.  -- B.E. Traver et al. / Journal of Invertebrate Pathology 109 (2012) 187–193

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