Because human communities and individuals are part of both local and global ecosystems, health conditions within them are influenced to a significant degree by climatic fluctuation. Changes in microbial ecosystems, the source of disease among human and animal populations, are often the direct result of variability of water. This paper considers three aspects of the interaction between water, disease, and human populations. The three aspects are temperature (in particular the significant warming of the past decades), drought, and overabundance of water. The three variables are intimately related and, as with any other aspects of ecosystems, cannot be easily separated from one another.