Background
The authors say "it is assumed that global warming is forced by the anthropogenic release of 'greenhouse gases'," and that a further "consistent assumption" has been a consequent "increased exposure of humans to tropical pathogens and their vectors." However, they also note "there is dissent about this hypothesis (Taubes, 1997; Reiter, 2001; Hay et al., 2002; Reiter et al., 2003; Randolph, 2004; Zell, 2004; Halstead, 2008)," and they thus go on to explore it in a bit more detail.

What was done
Zell et al. reviewed the pertinent literature, describing "those mechanisms that have lead to an increase of virus activity in recent years."

What was learned
The three German researchers found that "only very few examples point toward global warming as a cause of excess viral activity." Instead, they determined that "coupled ocean/atmosphere circulations and continuous anthropogenic disturbances (increased populations of humans and domestic animals, socioeconomic instability, armed conflicts, displaced populations, unbalanced ecosystems, dispersal of resistant pathogens etc.) appear to be the major drivers of disease variability," and that "global warming at best [our italics] contributes."

What it means
By far and away the most significant cause of enhanced viral diseases in recent years has not been global warming. In fact, two whole classes of factors have dominated this aspect of human health and wellbeing during this period.

References
Halstead, S.B. 2008. Dengue virus-mosquito interactions. Annual Review of Entomology 53: 273-291.

Hay, S.I., Rogers, D.J., Randolph, S.E., Stern, D.I., Cox, J., Shanks, G.D. and Snow, R.W. 2002. Hot topic or hot air? Climate change and malaria resurgence in East African highlands. Trends in Parasitology 18: 530-534.

Randolph, S.E. 2004. Evidence that climate change has caused 'emergence' of tick-borne diseases in Europe? International Journal of Medical Microbiology 293 (Supplement 37): 5-15.

Reiter, P. 2001. Climate change and mosquito-borne disease. Environmental Health Perspectives 109: 141-161.

Reiter, P., Lathrop, S., Bunning, M., Biggerstaff, B., Singer, D., Tiwari, T., Baber, L., Amador, M., Thirion, J., Hayes, J., Seca, C., Mendez, J., Ramirez, B., Robinson, J., Rawlings, J., Vorndam, V., Waterman, S., Gubler, D., Clark, G. and Hayes, E. 2003. Texas lifestyle limits transmission of dengue virus. Emerging Infectious Diseases 9: 86-89.

Taubes, G. 1997. Global warming: apocalypse not. Science 278: 1004-1006.

Zell, R. 2004. Global climate change and the emergence/re-emergence of infectious diseases. International Journal of Medical Microbiology 293 (Supplement 37): 16-26.