Which of the following is not a water-borne disease?

Concept

A disease is classified by the route through which its pathogen reaches a new host. A water-borne disease is one whose pathogen is swallowed with water — or with food washed or prepared in it — that carries human or animal faecal matter; this is the faecal-oral route. A vector-borne disease is one whose pathogen is carried from host to host by a living arthropod such as a mosquito, and here water can act only as the vector's breeding site, never as the medium that delivers the pathogen into the body.

Applying the test to each disease

Typhoid, hepatitis A and E, and cholera all reach the body by the faecal-oral route, so each satisfies the definition of a water-borne disease. Dengue is not transmitted by drinking contaminated water at all; it ordinarily reaches the body through the bite of an infected Aedes mosquito, and the rare non-vector routes on record — infected blood, a transplanted organ, or mother to child — do not involve water either. Dengue is therefore the disease that is not water-borne.

Cross-check

• Dengue does have a link with water, and that is what makes it easy to misplace: Aedes aegypti lays its eggs in small collections of clean, stagnant water such as coolers, overhead tanks, discarded tyres and flowerpot trays. That water breeds the vector; it does not carry the virus to anyone who drinks it.

• Hepatitis is a group of liver infections caused by different viruses. The enteric types A and E spread through sewage-contaminated water and food, whereas types B and C spread through blood and body fluids; only the enteric types belong in a list of water-borne diseases.

• A quick check: if boiling and chlorinating the drinking supply would break the chain of transmission, the disease is water-borne. That works for typhoid, hepatitis A and E, and cholera, but not for dengue, which is controlled by mosquito management.