Hyderabad: The immediate danger in a flood is obvious: drowning, injuries and destruction of homes and infrastructure. But once the water begins to recede, another public-health threat can emerge.
The devastating flash floods that hit Nepal on August 26 have provided a stark example. The flooding affected communities along the Bhote Koshi and Trishuli river corridors, damaging homes, roads, bridges and health facilities.
What does WHO say?
WHO says around 10,000 households need immediate relief, while damaged water and sanitation systems and disrupted healthcare are increasing the risk of communicable diseases.
For communities recovering from floods, the concern is not necessarily that an outbreak will happen, but that conditions created by flooding can make certain diseases more likely to spread.
Contaminated water is the first concern
Floodwater can mix with sewage, human waste and other contaminants. If drinking-water systems are damaged or contaminated, the risk of diarrhoeal diseases, cholera, typhoid and other water- and food-borne infections can rise.
WHO has identified diarrhoeal illnesses, water- and food-borne diseases and vector-borne diseases among the health risks currently being monitored in flood-hit Nepal.
UNICEF has also warned that damaged water systems and sanitation infrastructure in Nepal are increasing children's exposure to disease. The agency is sending hygiene kits and other supplies to affected communities.
The concern is particularly acute when people are displaced into temporary shelters where access to clean water, toilets and handwashing facilities may be limited.
Leptospirosis: The floodwater infection
Another disease associated with flooding is leptospirosis, a bacterial infection transmitted through water or soil contaminated with the urine of infected animals, particularly rodents.
Flooding can force contaminated water into places where people live and work, increasing opportunities for exposure.
Research from Kerala following the devastating 2018 and 2019 floods found that leptospirosis cases increased during the post-flood period, with the association particularly evident after the heavier 2018 flooding.
This makes leptospirosis an important disease to monitor after major floods, particularly among people involved in cleaning homes, removing debris or working in contaminated water and mud.
Mosquitoes can become a second problem
Flooding can also alter mosquito habitats. Once water remains stagnant in containers, pools and other areas, mosquito breeding can increase.
Depending on the location and pathogens already circulating, this can increase the risk of dengue, malaria and other vector-borne diseases.
WHO lists both water-borne and vector-borne diseases among the medium- and long-term health consequences associated with floods.
However, a flood does not automatically cause a dengue or malaria outbreak. Transmission depends on mosquito populations, local disease circulation, environmental conditions and the effectiveness of disease surveillance.
Nepal has seen this pattern before
The current situation is not Nepal’s first experience of post-flood health risks.
After Nepal's major floods and landslides in September 2024, WHO reported increased concerns over water pollution and water-borne diseases, along with elevated risks of vector-borne diseases, respiratory and skin conditions and mental-health problems.
UNICEF’s reporting from an earlier Nepal flood response similarly described the period after floodwaters receded as a time when contaminated surroundings and poor sanitation could create conditions for cholera, typhoid and other water-borne diseases.
Why the risk continues after the flood
The current Nepal disaster shows why the health response cannot end with rescue operations.
WHO says the 2026 floods have damaged health facilities and made some hospitals inaccessible. It is now supporting disease surveillance, emergency medical care and continuity of essential health services while monitoring emerging public-health risks.
The same principle applies to flood-affected regions elsewhere: once the immediate emergency passes, authorities need to monitor drinking-water quality, sanitation, diarrhoeal illnesses, mosquito-borne diseases and other infections.
The flood may disappear within hours or days. The conditions it leaves behind can remain for weeks, and that is when the next health emergency can begin.
