Background
The authors write that the United Nation's 2005 Millennium Ecosystem Assessment "identified strong links between climate change and disease emergence," and they say that there is thus "a general presumption that climate change will enhance the spread of diseases and increase their occurrence."

What was done
Ibelings et al. examined this claim via an analysis of the dynamics of host-parasite interactions over a period of more than 30 years in the case of the freshwater diatom Asterionella formosa and two highly virulent chytrid parasites (Rhizophydium planktonicum and Zygorhizidium planktonicum) in Lake Maarsseveen, The Netherlands, over which period there was a significant warming trend that was most strongly expressed in the spring.

What was learned
The six scientists determined that in years with cold winters, a dense Asterionella bloom is followed by epidemic development of disease, as "high Asterionella densities greatly facilitate transmission of chytrid zoospores," but they say that "this sequence of events is absent in milder winters," because climate warming reduces the period in which water temperature remains cold, thereby "narrowing the window of opportunity for uninfected growth," so that "Asterionella continuously suffers from infection." The resulting population reduction thus allows other diatoms to dominate the lake; and in these mild-winter years "chytrid infections no longer reach epidemic levels." Consequently, as they describe it, climate warming affects both host and parasite in intricate ways, with "the host denied a bloom" and "the parasite denied an epidemic."

What it means
Ibelings et al. say their study "presents a scenario that runs counter to the general expectation of a 'warmer hence sicker world'," and that "the body of research on Asterionella and chytrid infections in Lake Maarsseveen demonstrates that climate warming is not invariably linked with disease emergence and indeed may result in the opposite -- warmer winters promote a reduction in disease," a view that is analogous to that put forth some years earlier by Hall et al. (2006). What is more, they note that "this diminution of disease promotes changes in the composition of the diatom spring community, leading to enhanced edibility of the spring phytoplankton," and they logically speculate that "this in turn may have positive effects on higher trophic levels."

Reference
Hall, S.R., Tessier, A.J., Duffy, M.A., Huebner, M. and Caceres, C.E. 2006. Warmer does not have to mean sicker: temperature and predators can jointly drive timing of epidemics. Ecology 87: 1684-1695.