Kausrud and colleagues examine whether changes in climatic conditions over the past 15 centuries have potentiated these main excursions of the plague bacterium from its Central Asian, ground-burrowing, rodent host populations into human populations. They have used impressively extensive and detailed historical data for Central Asia (Kazakh region). Even so, when delving back over so many centuries, surrogate measures of paleoclimatic conditions are necessary - in this case, tree-rings, glaciers and stalagmites. The authors have also had to make simplifying assumptions about the determinants of (and, hence, the proxy index for) the changeable level of sylvatic (rodent) plague activity over time.
This research task is complex, and some readers will find the analytic methods and the inferences drawn as challenging as they are innovative. The initial fine-tuning of the model to estimate (from climatic conditions and, hence, vegetation cover) the changeable level of sylvatic plague activity within rodent populations over past centuries was achieved by the empirical comparison of 10,000 (yes, 10,000) computer-generated model variants tested against 20th century Kazakh data. Each variant comprised a slightly differing combination of the several proxy measures of climatic conditions - temperature, rainfall, and monsoonal shifts - and their own many possible representations.
This study is timely, as the world community is now increasingly attentive to the likely impacts of human-induced climate change on the occurrence of infectious disease [4, 5]. For many infectious agents, the probability of transmission is influenced variously by temperature, rainfall, humidity and wind patterns. The relationships between these factors are sometimes complex, and estimates of the impact of climate change on infectious disease have been contested . Overall, though, it is certain that changes in climatic conditions will reset the boundaries, spatial and seasonal, on the transmission of many infectious diseases.
Salmonella food poisoning (diarrheal disease) occurs most often in summer, reflecting the faster proliferation of bacteria at higher temperatures. Cholera outbreaks occur more readily when coastal waters warm or when heavy rains cause flooding. The malarial parasite matures faster within the mosquito at warmer temperatures, and mosquito breeding, biting and survival are sensitive to temperature, surface water and humidity . The northern limits of schistosomiasis transmission in China are set by the mid-winter 'freezing zone' - at which temperature the pathogen's intermediate host, the water snail, cannot survive . In warmer waters, development of the schistosomiasis parasite within the snail can only occur above 15.4°C.
For bubonic plague it is entirely possible that the natural reservoir populations of rodents would have been affected by regional climate changes, including impacts on regional vegetation food sources. Such a relationship has been previously postulated . In the current work, Kausrud et al.  focus on the level of sylvatic plague activity in the multi-decadal period preceding each of the three major pandemic outbreaks of plague and the Manchurian epidemics of the early 20th century.