Waterborne disease
The diseases water carries, why chlorination was the most consequential public health intervention of the twentieth century, and what remains unsolved.
Before water treatment, waterborne disease was among the leading causes of death in industrialised cities. Typhoid mortality in cities that adopted chlorination fell by an order of magnitude within a few years. It is difficult to name a public health intervention with a larger effect.
The main pathogens
| Organism | Illness | Effective barrier |
|---|---|---|
| Vibrio cholerae | Cholera — severe, rapidly dehydrating diarrhoea | Chlorination; sanitation |
| Salmonella Typhi | Typhoid fever | Chlorination; sanitation |
| Escherichia coli O157 | Haemorrhagic colitis; haemolytic uraemic syndrome | Chlorination; source protection |
| Campylobacter | Gastroenteritis — the most common bacterial cause in high-income countries | Chlorination |
| Cryptosporidium | Cryptosporidiosis — prolonged diarrhoea | Filtration and UV. Chlorine does not work |
| Giardia | Giardiasis | Filtration and UV |
| Norovirus and rotavirus | Viral gastroenteritis | Chlorination at adequate contact; filtration |
| Hepatitis A and E | Viral hepatitis | Chlorination; sanitation |
| Legionella pneumophila | Legionnaires’ disease — inhaled, not drunk | Temperature control in building systems |
The wider categories
- Waterborne
- Transmitted by drinking contaminated water. Cholera, typhoid, cryptosporidiosis.
- Water-washed
- Caused by inadequate water for hygiene rather than by contaminated water. Trachoma, many skin and eye infections. More water, not cleaner water, is the answer.
- Water-based
- Involving an aquatic intermediate host. Schistosomiasis, guinea worm.
- Water-related vector
- Transmitted by insects that breed in or near water. Malaria, dengue.
The distinction matters for what to do. Water-washed disease responds to water quantity and access; waterborne disease responds to treatment; water-based disease responds to breaking the host cycle. Treating them all as "dirty water" problems leads to the wrong intervention.
What remains
Waterborne disease remains a major cause of death globally, concentrated among children under five and overwhelmingly in places without safely managed water and sanitation. Sanitation is the binding constraint more often than treatment: contaminated water is usually contaminated because sanitation upstream is inadequate.
In high-income countries the residual risk sits in specific places — distribution network ingress, private supplies, and building water systems where Legionella grows. Those are engineering and maintenance problems rather than treatment problems.
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- WHO / UNICEF Joint Monitoring Programme for Water Supply, Sanitation and Hygiene — JMP global database. Public but restricted · CC BY-NC-SA 3.0 IGO
- UK Health Security Agency — Waterborne infectious disease surveillance and guidance. Open — attribution required · Open Government Licence v3.0