**2. Causative agent**

The causative agent of cholera, *V. cholerae*, is a waterborne and foodborne gram-negative bacterium. *Vibrio cholerae* can cause global pandemics which makes it unique among the diarrheal pathogens [12]. The serotype O1 or O139 colonizes and multiplies within the human small intestine [13]. There are two biotypes of *V. cholerae* O1: Classical and El Tor. Each of these biotypes has two serotypes: Inaba and Ogawa. Both these biotypes cause similar symptoms, however, El Tor biotype mostly causes asymptomatic infection or causes a mild illness [9]. Until 1992, only *V. cholerae* serogroup O1 was responsible for causing epidemics. However, in 1992, a previously unrecognized serogroup of *V. cholerae*, labeled as O139, was found to be responsible for large outbreaks of cholera in India and Bangladesh [3]. Multiple antibioticresistant strains of *V. cholerae* have also emerged and the severity of the disease appears to be intensifying [1].

#### **2.1. Mode of transmission**

Cholera is transmitted by drinking water or eating food contaminated with the *V. cholerae*. Fecal contamination of water or foods may result in large epidemics. The disease may also be transmitted through eating contaminated raw or undercooked shellfish [9]. Before the late 1970s, it was believed that person-to-person transmission of cholera is the main mode of transmission. Now, *V. cholerae* is found in riverine, estuarine, and coastal waters throughout both temperate and tropical regions of the world. It is recognized as a component of coastal and estuarine microbial ecosystems [14].

#### **2.2. Incubation period**

The disease has a short incubation period of 18 hours to 5 days, and it can spread rapidly through a population [12].
