ABSTRACT
The science of epidemiology emerged in response to some of the epidemics spreading through Europe, specifically England and France, during the 1800s. Just about any introductory epidemiology text will cover the London cholera epidemic, recalling the way Dr. John Snow cleverly traced it back to a water pump on Broad Street in London. Cholera is a horrible intestinal illness brought about by a bacterial pathogen that became very well adapted to thrive within large, settled human populations. Cholera epidemics have swept through the so-called Old World numerous times in the past. Heterozygous individuals may have been more likely to survive the cholera epidemics of the 1800s; that may be how the allele became so prominent in some groups. There is some indication, too, that heterozygous individuals are more likely to survive typhoid fever and TB.
