- Heatwaves, floods, erratic rainfall and droughts are altering the conditions that drive infectious diseases.
- Poorly managed drainage, sewage and water infrastructure can amplify health risks during extreme weather.
- India is working on climate-health surveillance and early-warning systems, but gaps in data and infrastructure could limit its ability to predict and prevent outbreaks, experts say.
When extreme weather events occur, the damage is usually measured in collapsed buildings, damaged roads and flooded homes. But the aftermath also brings a less visible threat.
Heatwaves, droughts, erratic rainfall and flash floods, in combination or alone, also reshape the ecology of diseases. They can affect how long pathogens survive, change the distribution of disease-carrying insects and bring people, animals and pathogens into closer contact, increasing the risk of outbreaks in new places at unexpected times.
The latest assessment report by the Intergovernmental Panel on Climate Change has warned that heatwaves and extreme rainfall events are projected to increase in the coming years. A 2025 study by IPE Global, an international development consultancy, and Esri India, a GIS mapping company, warns that extreme weather could become more severe by 2030, with heatwaves projected to double in major cities and extreme rainfall events expected to rise by 43% across the country.
Health experts state that we need to factor in and prevent climate-related outbreaks rather than simply responding to them.
Health risks during monsoons and droughts
The World Meteorological Organisation (WMO) reports that the past decade has witnessed some of the worst floods on record. India, too, has experienced devastating floods, including those in Mumbai in 2025 and Kerala in 2018.
“Heavy rainfall and flooding create ideal conditions for the emergence and spread of infections,” shares Surya Prasad, a consultant physician at Manipal Hospital, Bengaluru. “Floodwater overwhelms sewage systems and mixes human and animal waste into water sources, while warmer temperatures help pathogens survive longer.”
Some bacterial diarrhoeal diseases often peak during warmer, wetter conditions. A 21-year study from Kolkata, covering data from 1999 to 2019, found a biannual pattern in cholera cases, with peaks coinciding with rising summer temperatures and the onset of the monsoon.
Hepatitis A and E often see a rise in cases during the monsoon and post-monsoon months. In Delhi and Mumbai, major hepatitis E outbreaks have been linked to sewage entering water supplies after heavy monsoon rains.
A study conducted in Chennai found that the risk of hospitalisation for gastrointestinal illnesses was 60% higher in the 15 days following an extreme rainfall event.
But recent studies have also found that climate change is making these seasonal patterns unpredictable; it can change when and where infections appear. “Earlier, the cases of Hepatitis A, Hepatitis E and cholera were largely seen during the high temperatures of the pre-monsoon season. But over the past 10 years, we have been seeing these infections more frequently, and throughout the year,” Prasad elaborated.
Floods can also change the way pathogens move between animals and people. Flash floods can drive rodents out of sewers, burrows and other hiding places, while contaminated water can carry pathogens into homes and other areas, Prasad explained.
Leptospirosis, a zoonotic disease, shows a strong seasonal pattern, with more than 85% of annual cases occurring during and immediately after the monsoon. The bacteria that cause the disease spread through the urine of infected animals and can survive in contaminated water or soil for weeks to months.
“Rats are the primary hosts of Leptospira. During the monsoon, flooding drives them out of sewers, burrows and other hiding places into homes and drains. Their urine can contaminate water, food and surfaces, increasing the risk of infection, particularly during floods in cities,” said Prasad.
According to Integrated Disease Surveillance Project data, after the 2018 flood, Kerala witnessed 1,318 leptospirosis cases, including 53 deaths.
“Extreme weather events, such as intense rain or those leading to a natural disaster, can change the risk of specific infectious diseases,” explained Gautam Menon, a professor of Physics and Biology at Ashoka University, and the former director of the university’s Centre for Climate Change and Sustainability.
The effects are not limited to waterborne infections. Changes in temperature and rainfall can alter the breeding, behaviour and distribution of mosquitoes and other disease vectors, Menon explained. Warmer conditions can allow mosquitoes to survive and breed for longer, potentially expanding the geographic and seasonal window for diseases such as dengue, malaria and Zika.
In the Himalayan foothills, rising temperatures and shifting rainfall patterns are allowing malaria-carrying Anopheles mosquitoes to persist in areas and at elevations previously considered too cold for them. In 2024, Pune reported surges in Zika virus infections following heavy monsoon rains and flooding.
It is not only excessive water that poses a health risk. Droughts and prolonged water shortages can intensify the risk of infection in different ways.
A 2023 study found that as water sources shrink, people, livestock and wildlife may increasingly congregate around the same limited sources, increasing the chances of pathogen transmission. Other studies have linked overcrowding with the increase of pathogen transmission. Communities may also be forced to rely on unsafe or alternative sources of water, including those shared with domestic animals, increasing opportunities for contact and zoonotic disease transmission.
Less water for washing, food preparation and sanitation creates further opportunities for infection. Water collected from tankers or communal taps stored in large containers for long periods creates new sites for disease vectors such as mosquitoes to breed.
Lack of infrastructure compounds the problem
Extreme weather does not create disease risks in isolation. In India, the consequences are often amplified by weaknesses in basic infrastructure.
Poor urban planning, construction on floodplains and the loss of natural drainage areas have reduced the space available for rainwater to drain, increasing surface runoff. Much of the infrastructure that is expected to absorb the shock of extreme weather events is designed around historical and largely stable rainfall patterns.
Until recently, India’s stormwater drains were designed under Central Public Health and Environmental Engineering Organisation (CPHEEO) guidelines to handle rainfall occurring once every one to two years. Where rainfall data were unavailable, drains were typically designed for maximum rainfall intensity of 25 mm an hour — well below the intensity of several extreme rainfall events witnessed in recent years. In the month of May, Bengaluru, Delhi, and Mumbai were battered with more than 100 mm of rain in 24 hours. Many other cities in the country still function on archaic and poorly managed infrastructure and receive 100 mm in a single day, leaving the drains clogged with silt, debris and city waste.
A 2022 Comptroller and Auditor General of India report notes that municipal authorities in many cities do not have updated maps of underground drainage networks. Some of these systems are also decades old. In Mumbai and Kolkata, parts of the underground drainage network are more than a century old.
The floods that have swept through Assam since July this year are another reminder of how infrastructure built in the past is struggling to cope with today’s climate extremes. The state has more than 4,800 kilometres of embankments, yet around 60% are estimated to need repairs.
“I do not think any part of India is safe from these consequences. Damaged water pipes and storage systems, and runoff from pit latrines contaminate groundwater, particularly in rural areas,” said Sitara S.R. Ajjampur, a senior professor of Microbiology and the head of The Wellcome Trust Research Laboratory, and an adjunct faculty at the Center for Public Health, Christian Medical College, Vellore.
“When people are displaced, ensuring safe water in relief camps can also be difficult, while overwhelmed health services can delay basic treatment such as rehydration.”The number of water-borne diseases in India underscore a gap in preparedness and the need for surveillance, infrastructure investment, climate-adaptive planning and the National Disaster Management Authority working with public health groups.
From responding to outbreaks to anticipating them
The Intergovernmental Panel on Climate Change (IPCC) warns that rising temperatures and extreme weather events are likely to increase the burden of waterborne diseases. In India, however, the consequences of these events are compounded by weaknesses in basic infrastructure.
“India’s exposure to climate-sensitive disease is unusually high: it has endemic malaria, dengue, chikungunya, Japanese encephalitis, cholera, leptospirosis, and kala-azar, so the gaps in its preparedness matter a great deal,” said Menon. “A fundamental gap is surveillance, which often detects outbreaks late. Integration between health data and environmental data is not well-developed,” he added.
The challenge is to move from treating climate-related diseases as a series of unexpected outbreaks to recognising it as a predictable and increasingly preventable consequence of a changing climate. India has already started building a framework to address this.
In 2018, the Indian government launched the National Action Plan for Climate Change and Human Health (NAPCCHH), which aims to strengthen preparedness through disease surveillance, early-warning systems, capacity building and climate-resilient healthcare infrastructure.
“We are halfway there. All states have established their own state and district-level teams and started preparing context-specific plans,” said Upasona Ghosh, an associate professor at the Public Health Foundation of India. “However, the programme has yet to gain its due relevance, as climate change impacts are deep-rooted and long-term.” She also added that health workforces and policymakers need a better understanding of the complex pathways of climate change impacts on public health.
National Centre for Disease Control (NCDC) has also developed district-level climate and health risk assessments, which require states to use daily surveillance data, local civil-registration data on temperature-related mortality and weather information to establish local heat-health thresholds.
A 2025 study by the Indian Institute of Tropical Meteorology developed a model for Pune to predict dengue outbreaks more than two months in advance. The study found that dengue risk was the highest when temperatures were above 27°C, with moderate rainfall and humidity between 60% and 78%. However, the researchers stressed that these climate thresholds are region-specific, but such models could help health authorities anticipate outbreaks and prepare in advance.
“We have health surveillance, weather and infectious disease data, but these datasets need to be integrated, and their quality also needs to improve,” pointed out Purvi Patel, an associate fellow for Adaptation and Resilience at Sustainable Futures Collaboratives. “What we need is continuous, integrated data, along with the capacity to analyse and interpret it.”
“We also need to ground-truth these data and build the capacity to use them for early action. The goal should be to prevent climate-related outbreaks, rather than simply responding,” she concluded.
Banner image: A woman wades through floodwaters to collect drinking water near her submerged house in Jorhat, Assam in August 2026. (AP Photo/Parikhit Saikia)
Read more: A zoonotic disease in the Himalayas that needs a closer look [Commentary]
Source: india.mongabay.com
