Severe flooding could trigger major outbreaks of certain illnesses by changing how people interact with contaminated water, animals and disease-carrying insects. Waterborne bacterial and viral infections, including cholera, dysentery, typhoid fever and hepatitis A or E, pose the most immediate threats to public health. These pathogens can spread through contaminated drinking water and cause different illnesses, including severe diarrhoea, dehydration and, in the case of hepatitis A and E, liver inflammation. Between 1 January and 15 February this year, cholera cases surged, with 4,320 cases and 56 deaths recorded after flooding events in southern African countries, namely Malawi, Mozambique, Namibia, Zambia and Zimbabwe.
Similarly, leptospirosis poses a severe zoonotic bacterial threat in standing floodwaters. Transmitted through the urine of infected rodents and livestock, these harmful bacteria enter the human body through minor skin abrasions, eyes, or mucous membranes. Without prompt antibiotic treatment, severe leptospirosis can progress rapidly, leading to potentially life-threatening liver damage, internal bleeding, and acute kidney failure. In the Philippines, authorities recorded at least 13,158 leptospirosis cases as of 7 October 2026. Around 72 percent of cases recorded in Metro Manila are from flood-related exposure. At least 662 flooding events have been recorded across Metro Manila this year. More recently in Thailand, officials warned of a heightened risk of leptospirosis as flooding conditions persisted, with 3,303 leptospirosis cases and 46 deaths recorded since the start of the year.
As floodwaters recede, standing water may increase mosquito breeding, potentially raising the risk of vector-borne diseases such as dengue, malaria, West Nile virus and Zika virus. These mosquito-borne illnesses can cause high fevers, neurological complications and joint pain. An increase in mosquito-borne disease cases may occur several weeks after flooding, depending on local conditions. A dengue outbreak is worsening in Bangladesh, as officials recorded 81,598 dengue-related hospitalisations and 249 deaths as of 1 October 2026, with the month of September being the deadliest month this year. This outbreak may be exacerbated by the flash floods and heavy rain that killed at least 50 people and affected more than a million people during mid-July. In South America, Peru recorded 78,912 dengue cases as of 23 October 2026, with the worst-affected areas being Piura, La Libertad and Lambayeque departments, where flooding occurs.
Exposure to floodwaters may also cause skin conditions such as contact dermatitis, as well as bacterial and fungal infections such as ringworm. Finally, trapped indoor moisture could encourage rapid mould growth. Exposure to mould spores can aggravate existing respiratory conditions and trigger asthma symptoms, particularly in vulnerable populations.
