**1. Introduction**

Cholera is an acute diarrheal illness, caused by the toxigenic strains of the bacterium *Vibrio cholerae* serogroups O1 or O139 [1, 2]. It is one of the important public health problems in Asia and Africa and causes substantial morbidity and mortality [3]. Since centuries, cholera has been a subject of interest for epidemiologists. The studies regarding cholera helped in the development of new epidemiological methods which led to the understanding of not only cholera transmission but also helped in the development of the science of infectious disease epidemiology [4]. The purpose of this chapter is to discuss this important infectious disease, i.e., its historical aspects, clinical features, epidemiology, and the outbreaks caused by cholera.

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Moreover, the preventive measures for cholera and methods of control of cholera outbreaks will also be discussed.

#### **1.1. Historical aspects of cholera**

For centuries, cholera remained one of the most horrific diseases [5]. It was first described by Hippocrates in the fifth century BC. Traditionally, the Ganges Delta region in Asia is consid‐ ered the home of cholera. It is believed that cholera spread throughout the world from this region. Several epidemics occurred in Asia during the fifteenth and eighteenth centuries. Seven major pandemics of cholera have occurred since 1817 [3, 6].

Historians believe that the impact of cholera epidemics on the cultural evolution of Western Europe, was far reaching and it altered the social matrix of European culture. During the nineteenth century cholera was not only considered a terrifying disease, but was also a challenge to national identity and national economy [7].

John Snow performed pioneer work on cholera in London in the 1800s. He established an association between cholera transmission and contaminated water [4]. He discovered the method of prevention and control of cholera by tracing its source back to drinking water. Due to his work on the Soho cholera outbreak in 1854, John Snow has become a legend in epidemi‐ ology [5]. Later, in 1883, Robert Koch described the causative agent for cholera as a curved bacillus, *V. cholerae* [3].

In 1892, a major cholera outbreak occurred in Germany, killing 10,000 people. It was found to be caused by a defect in the design in the German waste removal system [7]. Seven cholera pandemics occurred during the nineteenth and twentieth centuries. The seventh pandemic began in Indonesia in 1961, reached West Africa in 1970 and the Americas in 1991 [3, 6].

In the United States of America, the risk of cholera is very low. However, in 2005, the floods caused by Hurricane Katrina, created the fear of a cholera epidemic for the first time in a century. The US Centers for Disease Control and Surveillance had 11 confirmed cases of people becoming ill from *Vibrio* sp. infection. However, only one of those cases had *V. cholerae*, which was not from the two epidemic-causing serogroups, i.e., O1 and O139 [5].

#### **1.2. General burden of cholera on human population**

Cholera is an indicator of a lack of social development and is a global threat to public health. With rapidly increasing population in developing world, the populations living in unsanitary conditions are increasing and the re-emergence of cholera has also been noted [8].

Every year, approximately 3–5 million cholera cases occur, worldwide. An estimated 100,000– 120,000 deaths due to cholera occur every year. The number of cholera cases reported to the World Health Organization (WHO) continues to rise. In 2011, a total of 589,854 cholera cases including 7816 cholera deaths were reported from 58 countries, with a case-fatality rate of 1.3% [8, 9]. A total of 838,315 cases belonging to the period 2004–2008 had been notified to the WHO, as compared to 676,651 cases notified from 2000 to 2004 [1]. In 2006, 52 countries reported 236,896 cholera cases including 6311 deaths with a case fatality rate of 2.7% [10].

The above-mentioned numbers are an underestimation of the real picture as the true number of cholera cases is thought to be much higher. Many cases may not be reported due to limitations in health-care systems and the surveillance systems. There are inconsistencies in cholera case definitions and there is a lack of standard vocabulary. Notified cases are often diagnosed on clinical grounds, leading to a number of undiagnosed and unreported cases. Some countries notify only laboratory-confirmed cases. It is a challenge in the estimation of disease burden due to the dearth of microbiology laboratories capable of detecting *V. choler‐ ae* O1 and O139 in the countries where the disease is prevalent. Fear of sanctions regarding travel and trade may also result in underreporting of cholera cases [10, 11].
