Which one of the following is a water borne disease?

Cholera is the answer. It is caused by Vibrio cholerae, which spreads through faecal contamination of drinking water and food. The bacterium colonises the small intestine and its toxin causes severe watery diarrhoea and dehydration. The core concept is the faecal-oral route of transmission.

• Small pox: caused by the variola virus and spread by respiratory droplets and close contact, not water. WHO declared it eradicated in 1980.
• Malaria: a vector-borne disease caused by Plasmodium and transmitted by the female Anopheles mosquito. Water matters only as a breeding ground for the vector, not as the route of infection.
• Tuberculosis: caused by Mycobacterium tuberculosis and spread through airborne droplets from an infected person's cough or sneeze.

Mains angle: waterborne diseases link directly to GS-II and GS-III on safe drinking water, the Jal Jeevan Mission, and sanitation under Swachh Bharat. ORS and zinc under diarrhoea control programmes are the public health response.

Trap: candidates pick malaria because water breeds mosquitoes. The question tests the mode of transmission of the causative organism, not whether water is present somewhere in its life cycle.