The idea that climate change is linked to the spread of a disease is not new. Some bacteria and viruses, after all, piggyback on an animal or insect, and the infectious advance depends on the host’s reaction to climbing temperatures. Consider dengue, a disease once anchored to tropical climates by its host’s penchant for heat and humidity, which is now pushing further north with its mosquito transits as the upper latitudes get warmer. But according to a study published this past June in PNAS, it’s not only climbing temperatures that are worrisome; in the past, even heavy rains have altered the course of disease, though often in divergent directions.

During the third plague pandemic (China, 1850-1964), researchers found that, for better or worse, the seasonal rains were a strong predictor of how the disease spread. There, storms governed Pestilence’s toll, prodding the disease in the arid north, and quelling it in the humid south.

Rats are the primary host for the bubonic plague, and in general, the more that infected rats move, the more the disease will spread. In the dry north, they figure, the rains quenched the parched landscape, causing the rats, and the disease, to stir. In the southern part of the country, the rains only served to make the humidity worse, perhaps forcing the rats to sit tight.

Keeping tabs on the spread of infectious disease is one thing; understanding the interaction of pathogens, hosts, and behavior is yet another.

Photo via Flickr / Yorick_R