Many infectious diseases—including malaria, Lyme disease, dengue fever, and cholera—are climate sensitive. In some cases, such as with malaria and dengue fever, this is in part because the disease is transmitted by mosquitoes which cannot survive if temperatures are too low. For others, climate restricts where an infection can occur because it limits the distribution of other species that are required for disease transmission.
Although some evidence indicates that warming may be causing malaria, for instance, to spread to higher elevations on mountains in East Africa, predicting how climate change will ultimately influence the incidence of diseases transmitted by insects remains a challenge. Consider that malaria was once common over much of North America and Europe in the 19th century but is not routinely present on either continent today, even after the temperature has warmed in the intervening century.
More predictable as climate change unfolds is the spread of waterborne infections. These infections most often cause diarrheal illness and flourish in the wake of heavy rainfalls as runoff from land enters into and may contaminate water supplies. Many pathogens that cause diarrheal disease reproduce more quickly in warmer conditions as well.